]> An in-depth exploration of the art of shell scripting Mendel Cooper Brindlesoft

thegrendel@theriver.com

1.2 31 March 2002 0.1 14 June 2000 mc Initial release. 0.2 30 October 2000 mc Bugs fixed, plus much additional material and more example scripts. 0.3 12 February 2001 mc Another major update. 0.4 08 July 2001 mc More bugfixes, much more material, more scripts - a complete revision and expansion of the book. 0.5 03 September 2001 mc Major update. Bugfixes, material added, chapters and sections reorganized. 1.0 14 October 2001 mc Bugfixes, reorganization, material added. Stable release. 1.1 06 January 2002 mc Bugfixes, material and scripts added. 1.2 31 March 2002 mc More bugfixes, material and scripts added. This tutorial assumes no previous knowledge of scripting or programming, but progresses rapidly toward an intermediate/advanced level of instruction (...all the while sneaking in little snippets of UNIX wisdom and lore). It serves as a textbook, a manual for self-study, and a reference and source of knowledge on shell scripting techniques. The exercises and heavily-commented examples invite active reader participation, under the premise that the only way to really learn scripting is to write scripts. The latest update of this document, as an archived tarball including both the SGML source and rendered HTML, may be downloaded from the author's home site. See the change log for a revision history. For Anita, the source of all the magic The shell is a command interpreter. More than just the insulating layer between the operating system kernel and the user, it's also a fairly powerful programming language. A shell program, called a script, is an easy-to-use tool for building applications by gluing together system calls, tools, utilities, and compiled binaries. Virtually the entire repertoire of UNIX commands, utilities, and tools is available for invocation by a shell script. If that were not enough, internal shell commands, such as testing and loop constructs, give additional power and flexibility to scripts. Shell scripts lend themselves exceptionally well to to administrative system tasks and other routine repetitive jobs not requiring the bells and whistles of a full-blown tightly structured programming language. A working knowledge of shell scripting is essential to everyone wishing to become reasonably adept at system administration, even if they do not anticipate ever having to actually write a script. Consider that as a Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and set up services. A detailed understanding of these startup scripts is important for analyzing the behavior of a system, and possibly modifying it. Writing shell scripts is not hard to learn, since the scripts can be built in bite-sized sections and there is only a fairly small set of shell-specific operators and options These are referred to as builtins, features internal to the shell. to learn. The syntax is simple and straightforward, similar to that of invoking and chaining together utilities at the command line, and there are only a few rules to learn. Most short scripts work right the first time, and debugging even the longer ones is straightforward. A shell script is a quick and dirty method of prototyping a complex application. Getting even a limited subset of the functionality to work in a shell script, even if slowly, is often a useful first stage in project development. This way, the structure of the application can be tested and played with, and the major pitfalls found before proceeding to the final coding in C, C++, Java, or Perl. Shell scripting hearkens back to the classical UNIX philosophy of breaking complex projects into simpler subtasks, of chaining together components and utilities. Many consider this a better, or at least more esthetically pleasing approach to problem solving than using one of the new generation of high powered all-in-one languages, such as Perl, which attempt to be all things to all people, but at the cost of forcing you to alter your thinking processes to fit the tool. When not to use shell scripts resource-intensive tasks, especially where speed is a factor (sorting, hashing, etc.) procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary precision calculations, or complex numbers (use C++ or FORTRAN instead) cross-platform portability required (use C instead) complex applications, where structured programming is a necessity (need typechecking of variables, function prototypes, etc.) mission-critical applications upon which you are betting the ranch, or the future of the company situations where security is important, where you need to guarantee the integrity of your system and protect against intrusion, cracking, and vandalism project consists of subcomponents with interlocking dependencies extensive file operations required (Bash is limited to serial file access, and that only in a particularly clumsy and inefficient line-by-line fashion) need multi-dimensional arrays need data structures, such as linked lists or trees need to generate or manipulate graphics or GUIs need direct access to system hardware need port or socket I/O need to use libraries or interface with legacy code proprietary, closed-source applications (shell scripts are necessarily Open Source) If any of the above applies, consider a more powerful scripting language, perhaps Perl, Tcl, Python, or possibly a high-level compiled language such as C, C++, or Java. Even then, prototyping the application as a shell script might still be a useful development step. We will be using Bash, an acronym for Bourne-Again Shell and a pun on Stephen Bourne's now classic Bourne Shell. Bash has become a de facto standard for shell scripting on all flavors of UNIX. Most of the principles dealt with in this book apply equally well to scripting with other shells, such as the Korn Shell, from which Bash derives some of its features, Many of the features of ksh88, and even a few from the updated ksh93 have been merged into Bash. and the C Shell and its variants. (Note that C Shell programming is not recommended due to certain inherent problems, as pointed out in a news group posting by Tom Christiansen in October of 1993). The following is a tutorial in shell scripting. It relies heavily on examples to illustrate features of the shell. As far as possible, the example scripts have been tested, and some of them may actually be useful in real life. The reader should use the actual examples in the the source archive (something-or-other.sh), By convention, user-written shell scripts that are Bourne shell compliant generally take a name with a .sh extension. System scripts, such as those found in /etc/rc.d, do not follow this guideline. give them execute permission (chmod u+rx scriptname), then run them to see what happens. Should the source archive not be available, then cut-and-paste from the HTML, pdf, or text rendered versions. Be aware that some of the scripts below introduce features before they are explained, and this may require the reader to temporarily skip ahead for enlightenment. Unless otherwise noted, the book author wrote the example scripts that follow. In the simplest case, a script is nothing more than a list of system commands stored in a file. At the very least, this saves the effort of retyping that particular sequence of commands each time it is invoked. &ex1; There is nothing unusual here, just a set of commands that could just as easily be invoked one by one from the command line on the console or in an xterm. The advantages of placing the commands in a script go beyond not having to retype them time and again. The script can easily be modified, customized, or generalized for a particular application. &ex2; Since you may not wish to wipe out the entire system log, this variant of the first script keeps the last section of the message log intact. You will constantly discover ways of refining previously written scripts for increased effectiveness. The sha-bang sha-bang ( #! #!) at the head of a script tells your system that this file is a set of commands to be fed to the command interpreter indicated. The #! is actually a two-byte Some flavors of UNIX (those based on 4.2BSD) take a four-byte magic number, requiring a blank after the !, #! /bin/sh. magic number magic number, a special marker that designates a file type, or in this case an executable shell script (see man magic for more details on this fascinating topic). Immediately following the sha-bang is a path name. This is the path to the program that interprets the commands in the script, whether it be a shell, a programming language, or a utility. This command interpreter then executes the commands in the script, starting at the top (line 1 of the script), ignoring comments. The #! line in a shell script will be the first thing the command interpreter (sh or bash) sees. Since this line begins with a #, it will be correctly interpreted as a comment when the command interpreter finally executes the script. The line has already served its purpose - calling the command interpreter. #!/bin/sh #!/bin/bash #!/usr/bin/perl #!/usr/bin/tcl #!/bin/sed -f #!/usr/awk -f Each of the above script header lines calls a different command interpreter, be it /bin/sh, the default shell (bash in a Linux system) or otherwise. This allows some cute tricks. #!/bin/rm # Self-deleting script. # Nothing much seems to happen when you run this... except that the file disappears. WHATEVER=65 echo "This line will never print (betcha!)." exit $WHATEVER # Doesn't matter. The script will not exit here. Also, try starting a README file with a #!/bin/more, and making it executable. The result is a self-listing documentation file. Using #!/bin/sh, the default Bourne Shell in most commercial variants of UNIX, makes the script portable to non-Linux machines, though you may have to sacrifice a few Bash-specific features (the script will conform to the POSIX Portable Operating System Interface, an attempt to standardize UNIX-like OSes. sh standard). Note that the path given at the sha-bang must be correct, otherwise an error message, usually Command not found will be the only result of running the script. #! can be omitted if the script consists only of a set of generic system commands, using no internal shell directives. Example 2, above, requires the initial #!, since the variable assignment line, lines=50, uses a shell-specific construct. Note that #!/bin/sh invokes the default shell interpreter, which defaults to /bin/bash on a Linux machine. This tutorial encourages a modular approach to constructing a script. Make note of and collect boilerplate code snippets that might be useful in future scripts. Eventually you can build a quite extensive library of nifty routines. As an example, the following script prolog tests whether the script has been invoked with the correct number of parameters. if [ $# -ne Number_of_expected args ] then echo "Usage: `basename $0` whatever" exit $WRONG_ARGS fi Having written the script, you can invoke it by sh scriptname, Caution: invoking a Bash script by sh scriptname turns off Bash-specific extensions, and the script may therefore fail to execute. or alternately bash scriptname. (Not recommended is using sh <scriptname, since this effectively disables reading from stdin within the script.) Much more convenient is to make the script itself directly executable with a chmod. Either: chmod 555 scriptname (gives everyone read/execute permission) A script needs read, as well as execute permission for it to run, since the shell needs to be able to read it. or chmod +rx scriptname (gives everyone read/execute permission) chmod u+rx scriptname (gives only the script owner read/execute permission) Having made the script executable, you may now test it by ./scriptname. Why not simply invoke the script with scriptname? If the directory you are in ($PWD) is where scriptname is located, why doesn't this work? This fails because, for security reasons, the current directory, . is not included in a user's $PATH. It is therefore necessary to explicitly invoke the script in the current directory with a ./scriptname. If it begins with a sha-bang line, invoking the script calls the correct command interpreter to run it. As a final step, after testing and debugging, you would likely want to move it to /usr/local/bin (as root, of course), to make the script available to yourself and all other users as a system-wide executable. The script could then be invoked by simply typing scriptname [ENTER] from the command line. System administrators often write scripts to automate common tasks. Give instances where such scripts would be useful. Write a script that upon invocation shows the time and date, lists all logged-in users, and gives the system uptime. The script then saves this information to a logfile. Chet Ramey ...there are dark corners in the Bourne shell, and people use all of them. The exit command exit exit command may be used to terminate a script, just as in a C program. It can also return a value, which is available to the script's parent process. Every command returns an exit status exit status (sometimes referred to as a return status return status ). A successful command returns a 0, while an unsuccessful one returns a non-zero value that usually may be interpreted as an error code. Well-behaved UNIX commands, programs, and utilities return a 0 exit code upon successful completion, though there are some exceptions. Likewise, functions within a script and the script itself return an exit status. The last command executed in the function or script determines the exit status. Within a script, an exit nnn command may be used to deliver an nnn exit status to the shell (nnn must be a decimal number in the 0 - 255 range). When a script ends with an exit that has no parameter, the exit status of the script is the exit status of the last command executed in the script (not counting the exit). $? variable $? $? reads the exit status of the last command executed. After a function returns, $? gives the exit status of the last command executed in the function. This is Bash's way of giving functions a return value. After a script terminates, a $? from the command line gives the exit status of the script, that is, the last command executed in the script, which is, by convention, 0 on success or an integer in the range 1 - 255 on error. &ex5; $? is especially useful for testing the result of a command in a script (see and ). The !, the logical not qualifier, reverses the outcome of a test or command, and this affects its exit status. true # the "true" builtin. echo "exit status of \"true\" = $?" # 0 ! true echo "exit status of \"! true\" = $?" # 1 # Note that the "!" needs a space. # !true leads to a "command not found" error # Thanks, S.C. Certain exit status codes have reserved meanings and should not be user-specified in a script. # # special character # comment Lines beginning with a # (with the exception of #!) are comments. # This line is a comment. Comments may also occur at the end of a command. echo "A comment will follow." # Comment here. Comments may also follow whitespace at the beginning of a line. # A tab precedes this comment. A command may not follow a comment on the same line. There is no method of terminating the comment, in order for live code to begin on the same line. Use a new line for the next command. Of course, an escaped # in an echo statement does not begin a comment. Likewise, a # appears in certain parameter substitution constructs and in numerical constant expressions. echo "The # here does not begin a comment." echo 'The # here does not begin a comment.' echo The \# here does not begin a comment. echo The # here begins a comment. echo ${PATH#*:} # Parameter substitution, not a comment. echo $(( 2#101011 )) # Base conversion, not a comment. # Thanks, S.C. The standard quoting and escape characters (" ' \) escape the #. Certain pattern matching operations also use the #. ; ; special character ; separator [Semicolon] Permits putting two or more commands on the same line. echo hello; echo there Note that the ; sometimes needs to be escaped. ;; ;; special character case ;; [Double semicolon] case "$variable" in abc) echo "$variable = abc" ;; xyz) echo "$variable = xyz" ;; esac . . special character . dot command source [period] Equivalent to source (see ). This is a bash builtin. In a different context, as part of a regular expression, a dot matches a single character. In yet another context, a dot is the filename prefix of a hidden file, a file that an ls will not normally show. bash$ touch .hidden-file bash$ ls -l total 10 -rw-r--r-- 1 bozo 4034 Jul 18 22:04 data1.addressbook -rw-r--r-- 1 bozo 4602 May 25 13:58 data1.addressbook.bak -rw-r--r-- 1 bozo 877 Dec 17 2000 employment.addressbook bash$ ls -al total 14 drwxrwxr-x 2 bozo bozo 1024 Aug 29 20:54 ./ drwx------ 52 bozo bozo 3072 Aug 29 20:51 ../ -rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.addressbook -rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.addressbook.bak -rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.addressbook -rw-rw-r-- 1 bozo bozo 0 Aug 29 20:54 .hidden-file " [double quote] "STRING" preserves (from interpretation) most of the special characters within STRING. See also . ' [single quote] 'STRING' preserves all special characters within STRING. This is a stronger form of quoting than using ". See also . , The comma operator links together a series of arithmetic operations. All are evaluated, but only the last one is returned. let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2". \ [backslash] \X escapes the character X. This has the effect of quoting X, equivalent to 'X'. The \ may be used to quote " and ', so they are expressed literally. See for an in-depth explanation of escaped characters. / [forward slash] Separates the components of a filename (as in /home/bozo/projects/Makefile). This is also the division arithmetic operator. ` [backticks] `command` makes available the output of command for setting a variable. This is also known as backticks or backquotes. : : special character : null command true endless loop [colon] This is the shell equivalent of a NOP (no op, a do-nothing operation). It may be considered a synonym for the shell builtin true. The : command is a Bash builtin, and its exit status is true (0). : echo $? # 0 Endless loop: while : do operation-1 operation-2 ... operation-n done # Same as: # while true # do # ... # done Placeholder in if/then test: if condition then : # Do nothing and branch ahead else take-some-action fi Provide a placeholder where a binary operation is expected, see and default parameters. : ${username=`whoami`} # ${username=`whoami`} without the leading : gives an error # unless "username" is a command or builtin... Provide a placeholder where a command is expected in a here document. See . Evaluate string of variables using parameter substitution (as in ). : ${HOSTNAME?} ${USER?} ${MAIL?} #Prints error message if one or more of essential environmental variables not set. Variable expansion / substring replacement. In combination with the > redirection operator, truncates a file to zero length, without changing its permissions. If the file did not previously exist, creates it. : > data.xxx # File "data.xxx" now empty. # Same effect as cat /dev/null >data.xxx # However, this does not fork a new process, since ":" is a builtin. See also . In combination with the >> redirection operator, updates a file access/modification time (: >> new_file). If the file did not previously exist, creates it. This is equivalent to touch. This applies to regular files, not pipes, symlinks, and certain special files. May be used to begin a comment line, although this is not recommended. Using # for a comment turns off error checking for the remainder of that line, so almost anything may be appear in a comment. However, this is not the case with :. : This is a comment that generates an error, ( if [ $x -eq 3] ). The : also serves as a field separator, in /etc/passwd, and in the $PATH variable. bash$ echo $PATH /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games ! ! special character ! not logical not The ! operator inverts the exit status of the command to which it is applied (see ). It also inverts the meaning of a test operator. This can, for example, change the sense of equal ( = ) to not-equal ( != ). The ! operator is a Bash keyword. In a different context, the ! also appears in indirect variable references. * * special character * wild card globbing regular expression [asterisk] The * character serves as a wild card for filename expansion in globbing, as well as representing any number (or zero) characters in a regular expression. A double asterisk, **, is the exponentiation operator. ? ? special character ? wild card globbing regular expression [question mark] The ? character serves as a single-character wild card for filename expansion in globbing, as well as representing one character in an extended regular expression. Within a double parentheses construct, the ? serves as a C-style trinary operator. See . $ $ special character $ variable substitution var1=5 var2=23skidoo echo $var1 # 5 echo $var2 # 23skidoo In a regular expression, a $ matches the end of a line. ${} ${} special character ${} parameter substitution $* $@ $* special character $* $@ positional parameters $@ () (a=hello; echo $a) A listing of commands within parentheses starts a subshell. Variables inside parentheses, within the subshell, are not visible to the rest of the script. The parent process, the script, cannot read variables created in the child process, the subshell. a=123 ( a=321; ) echo "a = $a" # a = 123 # "a" within parentheses acts like a local variable. Array=(element1 element2 element3) {xxx,yyy,zzz..} special character {} brace expansion {xxx,yyy,zzz,...} grep Linux file*.{txt,htm*} # Finds all instances of the work "Linux" # in the files "fileA.txt", "file2.txt", "fileR.html", "file-87.htm", etc. A command may act upon a comma-separated list of file specs within braces. The shell does the brace expansion. The command itself acts upon the result of the expansion. Filename expansion (globbing) applies to the file specs between the braces. No spaces allowed within the braces unless the spaces are quoted or escaped. echo {file1,file2}\ :{\ A," B",' C'} file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C {} {} special character {} block of code [curly brackets] Also referred to as an inline group, this construct, in effect, creates an anonymous function. However, unlike a function, the variables in a code block remain visible to the remainder of the script. bash$ { local a; a=123; } bash: local: can only be used in a function a=123 { a=321; } echo "a = $a" # a = 321 (value inside code block) # Thanks, S.C. The code block enclosed in braces may have I/O redirected to and from it. &ex8; &rpmcheck; Unlike a command group within (parentheses), as above, a code block enclosed by {braces} will not normally launch a subshell. Exception: a code block in braces as part of a pipe may be run as a subshell. ls | { read firstline; read secondline; } # Error. The code block in braces runs as a subshell, # so the output of "ls" cannot be passed to variables within the block. echo "First line is $firstline; second line is $secondline" # Will not work. # Thanks, S.C. {} \; Mostly used in find constructs. This is not a shell builtin. The ; ends the -exec option of a find command sequence. It needs to be escaped to protect it from interpretation by the shell. [ ] [] special character [ ] test Test expression between [ ]. Note that [ is part of the shell builtin test (and a synonym for it), not a link to the external command /usr/bin/test. [[ ]] [[]] special character [[ ]] test Test expression between [[ ]] (shell keyword). See the discussion on the [[ ... ]] construct. (( )) (( )) special character (( )) integer comparison Expand and evaluate integer expression between (( )). See the discussion on the (( ... )) construct. > >& >> < > >& >> < special character > special character >& special character >> special character < redirection scriptname >filename redirects the output of scriptname to file filename. Overwrite filename if it already exists. command >&2 redirects output of command to stderr. scriptname >>filename appends the output of scriptname to file filename. If filename does not already exist, it will be created. (command)> <(command) In a different context, the < and > characters act as string comparison operators. In yet another context, the < and > characters act as integer comparison operators. See also . << | | special character | pipe Passes the output of previous command to the input of the next one, or to the shell. This is a method of chaining commands together. echo ls -l | sh # Passes the output of "echo ls -l" to the shell, #+ with the same result as a simple "ls -l". cat *.lst | sort | uniq # Merges and sorts all ".lst" files, then deletes duplicate lines. A pipe, as a classic method of interprocess communication, sends the stdout of one process to the stdin of another. In a typical case, a command, such as cat or echo, pipes a stream of data to a filter for processing. cat $filename | grep $search_word The output of a command or commands may be piped to a script. #!/bin/bash # uppercase.sh : Changes input to uppercase. tr 'a-z' 'A-Z' # Letter ranges must be quoted #+ to prevent filename generation from single-letter filenames. exit 0 Now, let us pipe the output of ls -l to this script. bash$ ls -l | ./uppercase.sh -RW-RW-R-- 1 BOZO BOZO 109 APR 7 19:49 1.TXT -RW-RW-R-- 1 BOZO BOZO 109 APR 14 16:48 2.TXT -RW-R--R-- 1 BOZO BOZO 725 APR 20 20:56 DATA-FILE The stdout of each process in a pipe must be read as the stdin of the next. If this is not the case, the data stream will block, and the pipe will not behave as expected. cat file1 file2 | ls -l | sort # The output from "cat file1 file2" disappears. A pipe runs as a child process, and therefore cannot alter script variables. variable="initial_value" echo "new_value" | read variable echo "variable = $variable" # variable = initial_value If one of the commands in the pipe aborts, this prematurely terminates execution of the pipe. Called a broken pipe, this condition sends a SIGPIPE signal. >| >| special character >| redirection force noclobber This will forcibly overwrite an existing file. & A command followed by an & will run in the background. bash$ sleep 10 & [1] 850 [1]+ Done sleep 10 Within a script, commands and even loops may run in the background. &bgloop; A command run in the background within a script may cause the script to hang, waiting for a keystroke. Fortunately, there is a remedy for this. - - special character - redirection from/to stdin/stdout [dash] (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -) # Move entire file tree from one directory to another # [courtesy Alan Cox <a.cox@swansea.ac.uk>, with a minor change] # 1) cd /source/directory Source directory, where the files to be moved are. # 2) && "And-list": if the 'cd' operation successful, then execute the next command. # 3) tar cf - . The 'c' option 'tar' archiving command creates a new archive, # the 'f' (file) option, followed by '-' designates the target file as stdout, # and do it in current directory tree ('.'). # 4) | Piped to... # 5) ( ... ) a subshell # 6) cd /dest/directory Change to the destination directory. # 7) && "And-list", as above # 8) tar xpvf - Unarchive ('x'), preserve ownership and file permissions ('p'), # and send verbose messages to stdout ('v'), # reading data from stdin ('f' followed by '-'). # # Note that 'x' is a command, and 'p', 'v', 'f' are options. # Whew! # More elegant than, but equivalent to: # cd source-directory # tar cf - . | (cd ../target-directory; tar xzf -) # # cp -a /source/directory /dest also has same effect. bunzip2 linux-2.4.3.tar.bz2 | tar xvf - # --uncompress tar file-- | --then pass it to "tar"-- # If "tar" has not been patched to handle "bunzip2", # this needs to be done in two discrete steps, using a pipe. # The purpose of the exercise is to unarchive "bzipped" kernel source. Note that in this context the - is not itself a Bash operator, but rather an option recognized by certain UNIX utilities that write to stdout, such as tar, cat, etc. bash$ echo "whatever" | cat - whatever Where a filename is expected, - redirects output to stdout (sometimes seen with tar cf), or accepts input from stdin, rather than from a file. This is a method of using a file-oriented utility as a filter in a pipe. bash$ file Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file... By itself on the command line, file fails with an error message. bash$ file - #!/bin/bash standard input: Bourne-Again shell script text executable This time, it accepts input from stdin and filters it. The - can be used to pipe stdout to other commands. This permits such stunts as prepending lines to a file. Using diff to compare a file with a section of another: grep bash file1 | diff file2 - Finally, a real-world example using - with tar. &ex58; Filenames beginning with - may cause problems when coupled with the - redirection operator. A script should check for this and pass such filenames as ./-FILENAME or $PWD/-FILENAME. If the value of a variable begins with a -, this may likewise create problems. var="-n" echo $var # Has the effect of "echo -n", and outputs nothing. - [dash] cd - changes to previous working directory. This uses the $OLDPWD environmental variable. This is not to be confused with the - redirection operator just discussed. The interpretation of the - depends on the context in which it appears. - Minus sign in an arithmetic operation. = Assignment operator a=28 echo $a # 28 In a different context, the = is a string comparison operator. + Addition arithmetic operator. In a different context, the + is a Regular Expression operator. % Modulo (remainder of a division) arithmetic operation. In a different context, the % is a pattern matching operator. ~ [tilde] This corresponds to the $HOME internal variable. ~bozo is bozo's home directory, and ls ~bozo lists the contents of it. ~/ is the current user's home directory, and ls ~/ lists the contents of it. bash$ echo ~bozo /home/bozo bash$ echo ~ /home/bozo bash$ echo ~/ /home/bozo/ bash$ echo ~: /home/bozo: bash$ echo ~nonexistent-user ~nonexistent-user ~+ This corresponds to the $PWD internal variable. ~- This corresponds to the $OLDPWD internal variable. Control Characters A control character is a CONTROL + key combination. Ctl-C Terminate a foreground job. Ctl-D Log out from a shell (similar to exit). EOF (end of file). This also terminates input from stdin. Ctl-G BEL (beep). Ctl-H Backspace. #!/bin/bash # Embedding Ctl-H in a string. a="^H^H" # Two Ctl-H's (backspaces). echo "abcdef" # abcdef echo -n "abcdef$a " # abcd f # Space at end ^ ^ Backspaces twice. echo -n "abcdef$a" # abcdef # No space at end Doesn't backspace (why?). # Results may not be quite as expected. echo; echo Ctl-J Carriage return. Ctl-L Formfeed (clear the terminal screen). This has the same effect as the clear command. Ctl-M Newline. Ctl-U Erase a line of input. Ctl-Z Pause a foreground job. Whitespace Whitespace consists of either spaces, tabs, blank lines, or any combination thereof. In some contexts, such as variable assignment, whitespace is not permitted, and results in a syntax error. Blank lines have no effect on the action of a script, and are therefore useful for visually separating functional sections. $IFS, the special variable separating fields of input to certain commands, defaults to whitespace. Variables are at the heart of every programming and scripting language. They appear in arithmetic operations and manipulation of quantities, string parsing, and are indispensable for working in the abstract with symbols - tokens that represent something else. A variable is nothing more than a location or set of locations in computer memory holding an item of data. The name of a variable is a placeholder for its value, the data it holds. Referencing its value is called variable substitution. $ $ variable $ variable substitution Let us carefully distinguish between the name of a variable and its value. If variable1 is the name of a variable, then $variable1 is a reference to its value, the data item it contains. The only time a variable appears naked, without the $ prefix, is when declared or assigned, when unset, when exported, or in the special case of a variable representing a signal (see ). Assignment may be with an = (as in var1=27), in a read statement, and at the head of a loop (for var2 in 1 2 3). Enclosing a referenced value in double quotes (" ") does not interfere with variable substitution. This is called partial quoting, sometimes referred to as weak quoting. Using single quotes (' ') causes the variable name to be used literally, and no substitution will take place. This is full quoting, sometimes referred to as strong quoting. See for a detailed discussion. Note that $variable is actually a simplified alternate form of ${variable}. In contexts where the $variable syntax causes an error, the longer form may work (see , below). &ex9; An uninitialized variable has a null value - no assigned value at all (not zero!). Using a variable before assigning a value to it will inevitably cause problems. = = variable assignment the assignment operator (no space before & after) Do not confuse this with = and -eq, which test, rather than assign! Note that = can be either an assignment or a test operator, depending on context. &ex15; &ex16; Variable assignment using the $(...) mechanism (a newer method than backquotes) # From /etc/rc.d/rc.local R=$(cat /etc/redhat-release) arch=$(uname -m) Unlike many other programming languages, Bash does not segregate its variables by type. Essentially, Bash variables are character strings, but, depending on context, Bash permits integer operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits. &intorstring; Untyped variables are both a blessing and a curse. They permit more flexibility in scripting (enough rope to hang yourself) and make it easier to grind out lines of code. However, they permit errors to creep in and encourage sloppy programming habits. The burden is on the programmer to keep track of what type the script variables are. Bash will not do it for you. local variables variable local variables visible only within a code block or function (see also local variables in functions) environmental variables variable environmental variables that affect the behavior of the shell and user interface In a more general context, each process has an environment, that is, a group of variables that hold information that the process may reference. In this sense, the shell behaves like any other process. Every time a shell starts, it creates shell variables that correspond to its own environmental variables. Updating or adding new shell variables causes the shell to update its environment, and all the shell's child processes (the commands it executes) inherit this environment. The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems. bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`" bash$ du bash: /usr/bin/du: Argument list too long (Thank you, S. C. for the clarification, and for providing the above example.) If a script sets environmental variables, they need to be exported, that is, reported to the environment local to the script. This is the function of the export command. A script can export variables only to child processes, that is, only to commands or processes which that particular script initiates. A script invoked from the command line cannot export variables back to the command line environment. Child processes cannot export variables back to the parent processes that spawned them. --- positional parameters parameter positional arguments passed to the script from the command line - $0, $1, $2, $3... $0 is the name of the script itself, $1 is the first argument, $2 the second, $3 the third, and so forth. The process calling the script sets the $0 parameter. By convention, this parameter is the name of the script. See the manpage for execv. After $9, the arguments must be enclosed in brackets, for example, ${10}, ${11}, ${12}. &ex17; Some scripts can perform different operations, depending on which name they are invoked with. For this to work, the script needs to check $0, the name it was invoked by. There must also exist symbolic links to all the alternate names of the script. If a script expects a command line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent this is to append an extra character to both sides of the assignment statement using the expected positional parameter. variable1_=$1_ # This will prevent an error, even if positional parameter is absent. critical_argument01=$variable1_ # The extra character can be stripped off later, if desired, like so. variable1=${variable1_/_/} # Side effects only if $variable1_ begins with "_". # This uses one of the parameter substitution templates discussed in Chapter 9. # Leaving out the replacement pattern results in a deletion. # A more straightforward way of dealing with this is #+ to simply test whether expected positional parameters have been passed. if [ -z $1 ] then exit $POS_PARAMS_MISSING fi --- &ex18; --- shift command shift The shift command reassigns the positional parameters, in effect shifting them to the left one notch. $1 <--- $2, $2 <--- $3, $3 <--- $4, etc. The old $1 disappears, but $0 (the script name) does not change. If you use a large number of positional parameters to a script, shift lets you access those past 10, although {bracket} notation also permits this. &ex19; The shift command also works on parameters passed to a function. See . " special character " ' special character ' quote \ special character \ escape Quoting means just that, bracketing a string in quotes. This has the effect of protecting special characters in the string from reinterpretation or expansion by the shell or shell script. (A character is special if it has an interpretation other than its literal meaning, such as the wild card character, *.) bash$ ls -l [Vv]* -rw-rw-r-- 1 bozo bozo 324 Apr 2 15:05 VIEWDATA.BAT -rw-rw-r-- 1 bozo bozo 507 May 4 14:25 vartrace.sh -rw-rw-r-- 1 bozo bozo 539 Apr 14 17:11 viewdata.sh bash$ ls -l '[Vv]*' ls: [Vv]*: No such file or directory Certain programs and utilities can still reinterpret or expand special characters in a quoted string. This is an important use of quoting, protecting a command-line parameter from the shell, but still letting the calling program expand it. bash$ grep '[Ff]irst' *.txt file1.txt:This is the first line of file1.txt. file2.txt:This is the First line of file2.txt. Of course, grep [Ff]irst *.txt would not work. When referencing a variable, it is generally advisable in enclose it in double quotes (" "). This preserves all special characters within the variable name, except $, ` (backquote), and \ (escape). Keeping $ as a special character permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see , above). Use double quotes to prevent word splitting. Word splitting, in this context, means dividing a character string into a number of separate and discrete arguments. An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators. variable1="a variable containing five words" COMMAND This is $variable1 # Executes COMMAND with 7 arguments: # "This" "is" "a" "variable" "containing" "five" "words" COMMAND "This is $variable1" # Executes COMMAND with 1 argument: # "This is a variable containing five words" variable2="" # Empty. COMMAND $variable2 $variable2 $variable2 # Executes COMMAND with no arguments. COMMAND "$variable2" "$variable2" "$variable2" # Executes COMMAND with 3 empty arguments. COMMAND "$variable2 $variable2 $variable2" # Executes COMMAND with 1 argument (2 spaces). # Thanks, S.C. Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting is an issue. &weirdvars; Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally. Consider single quotes (full quoting) to be a stricter method of quoting than double quotes (partial quoting). Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result. echo "Why can't I write 's between single quotes" echo # The roundabout method. echo 'Why can'\''t I write '"'"'s between single quotes' # |-------| |----------| |-----------------------| # Three single-quoted strings, with escaped and quoted single quotes between. # This example courtesy of Stephane Chazelas. Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to interpret that character literally. With certain commands and utilities, such as echo and sed, escaping a character may have the opposite effect - it can toggle on a special meaning for that character. used with echo and sed \n \n escaped character \n newline means newline \r \r escaped character \r carriage return means return \t \t escaped character \t tabulation means tab \v \v escaped character \v vertical tabulation means vertical tab \b \b escaped character \b backspace means backspace \a \a escaped character \a alert beep flash means alert (beep or flash) \0xx \0xx escaped character \0xx octal ASCII translates to the octal ASCII equivalent of 0xx &escaped; See for another example of the $' ' string expansion construct. \" \" escaped character \" quote gives the quote its literal meaning echo "Hello" # Hello echo "\"Hello\", he said." # "Hello", he said. \$ \$ escaped character \$ dollar gives the dollar sign its literal meaning (variable name following \$ will not be referenced) echo "\$variable01" # results in $variable01 \\ \\ escaped character \\ double backslash gives the backslash its literal meaning echo "\\" # results in \ The behavior of \ depends on whether it is itself escaped, quoted, or appearing within command substitution or a here document. # Simple escaping and quoting echo \z # z echo \\z # \z echo '\z' # \z echo '\\z' # \\z echo "\z" # \z echo "\\z" # \z # Command substitution echo `echo \z` # z echo `echo \\z` # z echo `echo \\\z` # \z echo `echo \\\\z` # \z echo `echo \\\\\\z` # \z echo `echo \\\\\\\z` # \\z echo `echo "\z"` # \z echo `echo "\\z"` # \z # Here document cat <<EOF \z EOF # \z cat <<EOF \\z EOF # \z # These examples supplied by Stephane Chazelas. Elements of a string assigned to a variable may be escaped, but the escape character alone may not be assigned to a variable. variable=\ echo "$variable" # Will not work - gives an error message: # test.sh: : command not found # A "naked" escape cannot safely be assigned to a variable. # # What actually happens here is that the "\" escapes the newline and #+ the effect is variable=echo "$variable" #+ invalid variable assignment variable=\ 23skidoo echo "$variable" # 23skidoo # This works, since the second line #+ is a valid variable assignment. variable=\ # \^ escape followed by space echo "$variable" # space variable=\\ echo "$variable" # \ variable=\\\ echo "$variable" # Will not work - gives an error message: # test.sh: \: command not found # # First escape escapes second one, but the third one is left "naked", #+ with same result as first instance, above. variable=\\\\ echo "$variable" # \\ # Second and fourth escapes escaped. # This is o.k. Escaping a space can prevent word splitting in a command's argument list. file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7" # List of files as argument(s) to a command. # Add two files to the list, and list all. ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list echo "-------------------------------------------------------------------------" # What happens if we escape a couple of spaces? ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list # Error: the first three files concatenated into a single argument to 'ls -l' # because the two escaped spaces prevent argument (word) splitting. The escape also provides a means of writing a multi-line command. Normally, each separate line constitutes a different command, but an escape at the end of a line escapes the newline character, and the command sequence continues on to the next line. (cd /source/directory && tar cf - . ) | \ (cd /dest/directory && tar xpvf -) # Repeating Alan Cox's directory tree copy command, # but split into two lines for increased legibility. # As an alternative: tar cf - -C /source/directory | tar xpvf - -C /dest/directory # See note below. # (Thanks, Stephane Chazelas.) If a script line ends with a |, a pipe character, then a \, an escape, is not strictly necessary. It is, however, good programming practice to always escape the end of a line of code that continues to the following line. echo "foo bar" #foo #bar echo echo 'foo bar' # No difference yet. #foo #bar echo echo foo\ bar # Newline escaped. #foobar echo echo "foo\ bar" # Same here, as \ still interpreted as escape within weak quotes. #foobar echo echo 'foo\ bar' # Escape character \ taken literally because of strong quoting. #foor\ #bar # Examples suggested by Stephane Chazelas. if test if then else if elif Every reasonably complete programming language can test for a condition, then act according to the result of the test. Bash has the test command, various bracket and parenthesis operators, and the if/then construct. An if/then construct tests whether the exit status of a list of commands is 0 (since 0 means success by UNIX convention), and if so, executes one or more commands. There exists a dedicated command called [ (left bracket special character). It is a synonym for test, and a builtin for efficiency reasons. This command considers its arguments as comparison expressions or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for false). With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a command. Bash sees [[ $a -lt $b ]] as a single element, which returns an exit status. The (( ... )) and let ... constructs also return an exit status of 0 if the arithmetic expressions they evaluate expand to a non-zero value. These arithmetic expansion constructs may therefore be used to perform arithmetic comparisons. let "1<2" returns 0 (as "1<2" expands to "1") (( 0 && 1 )) returns 1 (as "0 && 1" expands to "0") An if can test any command, not just conditions enclosed within brackets. if cmp a b > /dev/null # Suppress output. then echo "Files a and b are identical." else echo "Files a and b differ." fi if grep -q Bash file then echo "File contains at least one occurrence of Bash." fi if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED then echo "Command succeeded." else echo "Command failed." fi An if/then construct can contain nested comparisons and tests. if echo "Next *if* is part of the comparison for the first *if*." if [[ $comparison = "integer" ]] then (( a < b )) else [[ $a < $b ]] fi then echo '$a is less than $b' fi This detailed if-test explanation courtesy of Stephane Chazelas. &ex10; Explain the behavior of , above. if [ condition-true ] then command 1 command 2 ... else # Optional (may be left out if not needed). # Adds default code block executing if original condition tests false. command 3 command 4 ... fi When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both if and then are keywords. Keywords (or commands) begin statements, and before a new statement on the same line begins, the old one must terminate. if [ -x "$filename" ]; then elif elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one. if [ condition1 ] then command1 command2 command3 elif [ condition2 ] # Same as else if then command4 command5 else default-command fi test test test [ special character [ ] special character ] The if test condition-true construct is the exact equivalent of if [ condition-true ]. As it happens, the left bracket, [ , is a token which invokes the test command. The closing right bracket, ] , in an if/test should not therefore be strictly necessary, however newer versions of Bash require it. The test command is a Bash builtin which tests file types and compares strings. Therefore, in a Bash script, test does not call the external /usr/bin/test binary, which is part of the sh-utils package. Likewise, [ does not call /usr/bin/[, which is linked to /usr/bin/test. bash$ type test test is a shell builtin bash$ type '[' [ is a shell builtin bash$ type '[[' [[ is a shell keyword bash$ type ']]' ]] is a shell keyword bash$ type ']' bash: type: ]: not found &ex11; test test test [[ special character [[ ]] special character ]] The [[ ]] construct is the shell equivalent of [ ]. This is the extended test command, adopted from ksh88. No filename expansion or word splitting takes place between [[ and ]], but there is parameter expansion and command substitution. file=/etc/passwd if [[ -e $file ]] then echo "Password file exists." fi Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example, The &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct. Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly necessary. dir=/home/bozo if cd "$dir" 2>/dev/null; then # "2>/dev/null" hides error message. echo "Now in $dir." else echo "Can't change to $dir." fi The "if COMMAND" construct returns the exit status of COMMAND. Similarly, a condition within test brackets may stand alone without an if, when used in combination with a list construct. var1=20 var2=22 [ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2" home=/home/bozo [ -d "$home" ] || echo "$home directory does not exist." test test test (( special character )) (( special character )) The (( )) construct expands and evaluates an arithmetic expression. If the expression evaluates as zero, it returns an exit status of 1, or false. A non-zero expression returns an exit status of 0, or true. This is in marked contrast to using the test and [ ] constructs previously discussed. &arithtests; -e file exists -f file is a regular file (not a directory or device file) -s file is not zero size -d file is a directory -b file is a block device (floppy, cdrom, etc.) -c file is a character device (keyboard, modem, sound card, etc.) -p file is a pipe -h file is a symbolic link -L file is a symbolic link -S file is a socket -t file (descriptor) is associated with a terminal device This test option may be used to check whether the stdin ([ -t 0 ]) or stdout ([ -t 1 ]) in a given script is a terminal. -r file has read permission (for the user running the test) -w file has write permission (for the user running the test) -x file has execute permission (for the user running the test) -g set-group-id (sgid) flag set on file or directory If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory, not necessarily to the group of the user who created the file. This may be useful for a directory shared by a workgroup. -u set-user-id (suid) flag set on file A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it. Be aware that suid binaries may open security holes and that the suid flag has no effect on shell scripts. This is useful for executables (such as pppd and cdrecord) that need to access system hardware. Lacking the suid flag, these binaries could not be invoked by a non-root user. -rwsr-xr-t 1 root 178236 Oct 2 2000 /usr/sbin/pppd A file with the suid flag set shows an s in its permissions. -k sticky bit set Commonly known as the sticky bit, the save-text-mode flag is a special type of file permission. If a file has this flag set, that file will be kept in cache memory, for quicker access. On modern UNIX systems, the sticky bit is no longer used for files, only on directories. If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing. drwxrwxrwt 7 root 1024 May 19 21:26 tmp/ If a user does not own a directory that has the sticky bit set, but has write permission in that directory, he can only delete files in it that he owns. This keeps users from inadvertently overwriting or deleting each other's files in a publicly accessible directory, such as /tmp. -O you are owner of file -G group-id of file same as yours -N file modified since it was last read f1 -nt f2 file f1 is newer than f2 f1 -ot f2 file f1 is older than f2 f1 -ef f2 files f1 and f2 are hard links to the same file ! not -- reverses the sense of the tests above (returns true if condition absent). , , , , and illustrate uses of the file test operators. -eq is equal to if [ "$a" -eq "$b" ] -ne is not equal to if [ "$a" -ne "$b" ] -gt is greater than if ["$a" -gt "$b" ] -ge is greater than or equal to if [ "$a" -ge "$b" ] -lt is less than if [ "$a" -lt "$b" ] -le is less than or equal to if [ "$a" -le "$b" ] < is less than (within double parentheses) (("$a" < "$b")) <= is less than or equal to (within double parentheses) (("$a" <= "$b")) > is greater than (within double parentheses) (("$a" > "$b")) >= is greater than or equal to (within double parentheses) (("$a" >= "$b")) = is equal to if [ "$a" = "$b" ] == is equal to if [ "$a" == "$b" ] This is a synonym for =. [[ $a == z* ]] # true if $a starts with an "z" (pattern matching) [[ $a == "z*" ]] # true if $a is equal to z* [ $a == z* ] # file globbing and word splitting take place [ "$a" == "z*" ] # true if $a is equal to z* # Thanks, S.C. != is not equal to if [ "$a" != "$b" ] This operator uses pattern matching within a [[ ... ]] construct. < is less than, in ASCII alphabetical order if [[ "$a" < "$b" ]] if [ "$a" \< "$b" ] Note that the < needs to be escaped within a [ ] construct. > is greater than, in ASCII alphabetical order if [[ "$a" > "$b" ]] if [ "$a" \> "$b" ] Note that the > needs to be escaped within a [ ] construct. See for an application of this comparison operator. -z string is null, that is, has zero length -n string is not null. The -n test absolutely requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see ) normally works, however, this is an unsafe practice. Always quote a tested string. As S.C. points out, in a compound test, even quoting the string variable might not suffice. [ -n "$string" -o "$a" = "$b" ] may cause an error with some versions of Bash if $string is empty. The safe way is to append an extra character to possibly empty variables, [ "x$string" != x -o "x$a" = "x$b" ] (the x's cancel out). &ex13; &strtest; &ex14; -a logical and exp1 -a exp2 returns true if both exp1 and exp2 are true. -o logical or exp1 -o exp2 returns true if either exp1 or exp2 are true. These are similar to the Bash comparison operators && and ||, used within double brackets. [[ condition1 && condition2 ]] The -o and -a operators work with the test command or occur within single test brackets. if [ "$exp1" -a "$exp2" ] Refer to and to see compound comparison operators in action. Condition tests using the if/then construct may be nested. The net result is identical to using the && compound comparison operator above. if [ condition1 ] then if [ condition2 ] then do-something # But only if both "condition1" and "condition2" valid. fi fi See for an example of nested if/then condition tests. The systemwide xinitrc file can be used to launch the X server. This file contains quite a number of if/then tests, as the following excerpt shows. if [ -f $HOME/.Xclients ]; then exec $HOME/.Xclients elif [ -f /etc/X11/xinit/Xclients ]; then exec /etc/X11/xinit/Xclients else # failsafe settings. Although we should never get here # (we provide fallbacks in Xclients as well) it can't hurt. xclock -geometry 100x100-5+5 & xterm -geometry 80x50-50+150 & if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then netscape /usr/share/doc/HTML/index.html & fi fi Explain the test constructs in the above excerpt, then examine the entire file, /etc/X11/xinit/xinitrc, and analyze the if/then test constructs there. You may need to refer ahead to the discussions of grep, sed, and regular expressions. variable assignment Initializing or changing the value of a variable = operation = = All-purpose assignment operator, which works for both arithmetic and string assignments. var=27 category=minerals # No spaces allowed after the "=". Do not confuse the = assignment operator with the = test operator. # = as a test operator if [ "$string1" = "$string2" ] # if [ "X$string1" = "X$string2" ] is safer, # to prevent an error message should one of the variables be empty. # (The prepended "X" characters cancel out.) then command fi expr command expr let command let + + operation + addition plus plus - - operation - subtraction minus minus * * operation * multiplication multiplication / / operation / division division ** ** operation ** exponentiation exponentiation # Bash, version 2.02, introduced the "**" exponentiation operator. let "z=5**3" echo "z = $z" # z = 125 % % operation % modulo modulo, or mod (returns the remainder of an integer division operation) bash$ echo `expr 5 % 3` 2 This operator finds use in, among other things, generating numbers within a specific range (see and ) and formatting program output (see and ). It can even be used to generate prime numbers, (see ). Modulo turns up surprisingly often in various numerical recipes. &gcd; += += operation += plus-equal plus-equal (increment variable by a constant) let "var += 5" results in var being incremented by 5. -= -= operation -= minus-equal minus-equal (decrement variable by a constant) *= *= operation *= times-equal times-equal (multiply variable by a constant) let "var *= 4" results in var being multiplied by 4. /= /= operation /= slash-equal slash-equal (divide variable by a constant) %= %= operation %= mod-equal mod-equal (remainder of dividing variable by a constant) Arithmetic operators often occur in an expr or let expression. &arithops; Integer variables in Bash are actually signed long (32-bit) integers, in the range of -2147483648 to 2147483647. An operation that takes a variable outside these limits will give an erroneous result. a=2147483646 echo "a = $a" # a = 2147483646 let "a+=1" # Increment "a". echo "a = $a" # a = 2147483647 let "a+=1" # increment "a" again, past the limit. echo "a = $a" # a = -2147483648 # ERROR (out of range) Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as strings. a=1.5 let "b = $a + 1.3" # Error. # t2.sh: let: b = 1.5 + 1.3: syntax error in expression (error token is ".5 + 1.3") echo "b = $b" # b=1 Use bc in scripts that that need floating point calculations or math library functions. The bitwise operators seldom make an appearance in shell scripts. Their chief use seems to be manipulating and testing values read from ports or sockets. Bit flipping is more relevant to compiled languages, such as C and C++, which run fast enough to permit its use on the fly. << << operation << left shift bitwise left shift (multiplies by 2 for each shift position) <<= <<= operation <<= left-shift-equal left-shift-equal let "var <<= 2" results in var left-shifted 2 bits (multiplied by 4) >> >> operation >> right shift bitwise right shift (divides by 2 for each shift position) >>= >>= operation >>= right-shift-equal right-shift-equal (inverse of <<=) & & operation & AND bitwise bitwise and &= &= operation &= and-equal bitwise and-equal | | operation | OR bitwise bitwise OR |= |= operation |= OR-equal bitwise OR-equal ~ ~ operation ~ negate bitwise negate ! ! operation ! NOT bitwise NOT ^ ^ operation ^ XOR bitwise XOR ^= ^= operation ^= XOR-equal bitwise XOR-equal && && operator && AND logical and (logical) if [ $condition1 ] && [ $condition2 ] # Same as: if [ $condition1 -a $condition2 ] # Returns true if both condition1 and condition2 hold true... if [[ $condition1 && $condition2 ]] # Also works. # Note that && operator not permitted within [ ... ] construct. && may also, depending on context, be used in an and list to concatenate commands. || || operator || OR logical or (logical) if [ $condition1 ] || [ $condition2 ] # Same as: if [ $condition1 -o $condition2 ] # Returns true if either condition1 or condition2 holds true... if [[ $condition1 || $condition2 ]] # Also works. # Note that || operator not permitted within [ ... ] construct. Bash tests the exit status of each statement linked with a logical operator. &andor; The && and || operators also find use in an arithmetic context. bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0)) 1 0 1 0 , , operation , linking comma operator The comma operator chains together two or more arithmetic operations. All the operations are evaluated (with possible side effects, but only the last operation is returned. let "t1 = ((5 + 3, 7 - 1, 15 - 4))" echo "t1 = $t1" # t1 = 11 let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2". echo "t2 = $t2 a = $a" # t2 = 5 a = 9 The comma operator finds use mainly in for loops. See . A shell script interprets a number as decimal (base 10), unless that number has a special prefix or notation. A number preceded by a 0 is octal (base 8). A number preceded by 0x is hexadecimal (base 16). A number with an embedded # is evaluated as BASE#NUMBER (this option is of limited usefulness because of range restrictions). &numbers; Used properly, variables can add power and flexibility to scripts. This requires learning their subtleties and nuances. Builtin variables variables affecting bash script behavior $BASH $BASH variable $BASH path to bash the path to the Bash binary itself, usually /bin/bash $BASH_ENV $BASH_ENV variable $BASH_ENV an environmental variable pointing to a Bash startup file to be read when a script is invoked $BASH_VERSINFO[n] $BASH_VERSINFO variable version information a 6-element array containing version information about the installed release of Bash. This is similar to $BASH_VERSION, below, but a bit more detailed. # Bash version info: for n in 0 1 2 3 4 5 do echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}" done # BASH_VERSINFO[0] = 2 # Major version no. # BASH_VERSINFO[1] = 04 # Minor version no. # BASH_VERSINFO[2] = 21 # Patch level. # BASH_VERSINFO[3] = 1 # Build version. # BASH_VERSINFO[4] = release # Release status. # BASH_VERSINFO[5] = i386-redhat-linux-gnu # Architecture # (same as $MACHTYPE). $BASH_VERSION $BASH_VERSION variable $BASH_VERSION the version of Bash installed on the system bash$ echo $BASH_VERSION 2.04.12(1)-release tcsh% echo $BASH_VERSION BASH_VERSION: Undefined variable. Checking $BASH_VERSION is a good method of determining which shell is running. $SHELL does not necessarily give the correct answer. $DIRSTACK $DIRSTACK variable $DIRSTACK directory stack directory stack the top value in the directory stack (affected by pushd and popd) This builtin variable corresponds to the dirs command, however dirs shows the entire contents of the directory stack. $EDITOR $EDITOR variable $EDITOR editor the default editor invoked by a script, usually vi or emacs. $EUID $EUID variable $EUID effective user id effective user id number Identification number of whatever identity the current user has assumed, perhaps by means of su. The $EUID is not necessarily the same as the $UID. $FUNCNAME $FUNCNAME variable function name name of the current function xyz23 () { echo "$FUNCNAME now executing." # xyz23 now executing. } xyz23 echo "FUNCNAME = $FUNCNAME" # FUNCNAME = # Null value outside a function. $GLOBIGNORE $GLOBIGNORE variable globbing ignore A list of filename patterns to be excluded from matching in globbing. $GROUPS $GROUPS variable $GROUPS groups groups current user belongs to This is a listing (array) of the group id numbers for current user, as recorded in /etc/passwd. root# echo $GROUPS 0 root# echo ${GROUPS[1]} 1 root# echo ${GROUPS[5]} 6 $HOME $HOME variable $HOME home directory directory home home directory of the user, usually /home/username (see ) $HOSTNAME $HOSTNAME variable $HOSTNAME system name variable name The hostname command assigns the system name at bootup in an init script. However, the gethostname() function sets the Bash internal variable $HOSTNAME. See also . $HOSTTYPE $HOSTTYPE variable $HOSTTYPE host type host type Like $MACHTYPE, identifies the system hardware. bash$ echo $HOSTTYPE i686 $IFS $IFS variable $IFS input field separator input field separator This defaults to whitespace (space, tab, and newline), but may be changed, for example, to parse a comma-separated data file. Note that $* uses the first character held in $IFS. See . bash$ echo $IFS | cat -vte $ bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"' w:x:y:z $IFS does not handle whitespace the same as it does other characters. &ifsh; (Thanks, S. C., for clarification and examples.) $IGNOREEOF $IGNOREEOF variable $IGNOREEOF Ignore EOF ignore EOF: how many end-of-files (control-D) the shell will ignore before logging out. $LC_COLLATE $LC_COLLATE variable $LC_COLLATE lowercase collate Often set in the .bashrc or /etc/profile files, this variable controls collation order in filename expansion and pattern matching. If mishandled, LC_COLLATE can cause unexpected results in filename globbing. As of version 2.05 of Bash, filename globbing no longer distinguishes between lowercase and uppercase letters in a character range between brackets. For example, ls [A-M]* would match both File1.txt and file1.txt. To revert to the customary behavior of bracket matching, set LC_COLLATE to C by an export LC_COLLATE=C in /etc/profile and/or ~/.bashrc. $LC_CTYPE $LC_CTYPE variable $LC_CTYPE lowercase character type This internal variable controls character interpretation in globbing and pattern matching. $LINENO $LINENO variable $LINENO line number This variable is the line number of the shell script in which this variable appears. It has significance only within the script in which it appears, and is chiefly useful for debugging purposes. last_cmd_arg=$_ # Save it. echo "At line number $LINENO, variable \"v1\" = $v1" echo "Last command argument processed = $last_cmd_arg" $MACHTYPE $MACHTYPE variable $MACHTYPE machine type machine type Identifies the system hardware. bash$ echo $MACHTYPE i686-debian-linux-gnu $OLDPWD $OLDPWD variable $OLDPWD previous working directory directory working old working directory (OLD-print-working-directory, previous directory you were in) $OSTYPE $OSTYPE variable $OSTYPE os type operating system type bash$ echo $OSTYPE linux-gnu $PATH $PATH variable $PATH path to binaries path to binaries, usually /usr/bin/, /usr/X11R6/bin/, /usr/local/bin, etc. When given a command, the shell automatically does a hash table search on the directories listed in the path for the executable. The path is stored in the environmental variable, $PATH, a list of directories, separated by colons. Normally, the system stores the $PATH definition in /etc/profile and/or ~/.bashrc (see ). bash$ echo $PATH /bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin PATH=${PATH}:/opt/bin appends the /opt/bin directory to the current path. In a script, it may be expedient to temporarily add a directory to the path in this way. When the script exits, this restores the original $PATH (a child process, such as a script, may not change the environment of the parent process, the shell). The current working directory, ./, is usually omitted from the $PATH as a security measure. $PIPESTATUS $PIPESTATUS variable pipe Exit status of last executed pipe. Interestingly enough, this does not give the same result as the exit status of the last executed command. bash$ echo $PIPESTATUS 0 bash$ ls -al | bogus_command bash: bogus_command: command not found bash$ echo $PIPESTATUS 141 bash$ ls -al | bogus_command bash: bogus_command: command not found bash$ echo $? 127 $PPID $PPID variable $PPID process id The $PPID of a process is the process id (pid) of its parent process. The pid of the currently running script is $$, of course. Compare this with the pidof command. $PS1 $PS1 variable $PS1 prompt This is the main prompt, seen at the command line. $PS2 $PS2 variable $PS2 prompt secondary The secondary prompt, seen when additional input is expected. It displays as >. $PS3 $PS3 variable $PS3 prompt tertiary The tertiary prompt, displayed in a select loop (see ). $PS4 $PS4 variable $PS4 prompt quartenary The quartenary prompt, shown at the beginning of each line of output when invoking a script with the -x option. It displays as +. $PWD $PWD variable $PWD working directory directory working working directory (directory you are in at the time) This is the analog to the pwd builtin command. &wipedir; $REPLY $REPLY variable $REPLY default value of read reply read The default value when a variable is not supplied to read. Also applicable to select menus, but only supplies the item number of the variable chosen, not the value of the variable itself. &reply; $SECONDS $SECONDS variable $SECONDS number of seconds the script has been running runtime seconds The number of seconds the script has been running. &seconds; $SHELLOPTS $SHELLOPTS variable $SHELLOPTS shell options the list of enabled shell options, a readonly variable $SHLVL $SHLVL variable $SHLVL shell level Shell level, how deeply Bash is nested. If, at the command line, $SHLVL is 1, then in a script it will increment to 2. $TMOUT $TMOUT variable $TMOUT timeout interval If the $TMOUT environmental variable is set to a non-zero value time, then the shell prompt will time out after time seconds. This will cause a logout. Unfortunately, this works only while waiting for input at the shell prompt console or in an xterm. While it would be nice to speculate on the uses of this internal variable for timed input, for example in combination with read, $TMOUT does not work in that context and is virtually useless for shell scripting. (Reportedly the ksh version of a timed read does work). Implementing timed input in a script is certainly possible, but may require complex machinations. One method is to set up a timing loop to signal the script when it times out. This also requires a signal handling routine to trap (see ) the interrupt generated by the timing loop (whew!). &tmdin; An alternative is using stty. &timeout; Perhaps the simplest method is using the -t option to read. &tout; $UID $UID variable $UID user id user id number current user's user identification number, as recorded in /etc/passwd This is the current user's real id, even if she has temporarily assumed another identity through su. $UID is a readonly variable, not subject to change from the command line or within a script, and is the counterpart to the id builtin. &amiroot; See also . The variables $ENV, $LOGNAME, $MAIL, $TERM, $USER, and $USERNAME are not Bash builtins. These are, however, often set as environmental variables in one of the Bash startup files. $SHELL, the name of the user's login shell, may be set from /etc/passwd or in an init script, and it is likewise not a Bash builtin. tcsh% echo $LOGNAME bozo tcsh% echo $SHELL /bin/tcsh tcsh% echo $TERM rxvt bash$ echo $LOGNAME bozo bash$ echo $SHELL /bin/tcsh bash$ echo $TERM rxvt $0, $1, $2, etc. $0 variable $0 positional parameter parameter positional positional parameters, passed from command line to script, passed to a function, or set to a variable (see and ) $# $# variable $# positional parameter number of parameter positional number of number of command line arguments The words argument and parameter are often used interchangeably. In the context of this document, they have the same precise meaning, that of a variable passed to a script or function. or positional parameters (see ) $* $* variable $* positional parameter all parameter positional all All of the positional parameters, seen as a single word $@ $@ variable $* positional parameter all Same as $*, but each parameter is a quoted string, that is, the parameters are passed on intact, without interpretation or expansion. This means, among other things, that each parameter in the argument list is seen as a separate word. &arglist; Following a shift, the $@ holds the remaining command-line parameters, lacking the previous $1, which was lost. #!/bin/bash # Invoke with ./scriptname 1 2 3 4 5 echo "$@" # 1 2 3 4 5 shift echo "$@" # 2 3 4 5 shift echo "$@" # 3 4 5 # Each "shift" loses parameter $1. # "$@" then contains the remaining parameters. The $@ special parameter finds use as a tool for filtering input into shell scripts. The cat "$@" construction accepts input to a script either from stdin or from files given as parameters to the script. See and . The $* and $@ parameters sometimes display inconsistent and puzzling behavior, depending on the setting of $IFS. &incompat; The $@ and $* parameters differ only when between double quotes. &ifsempty; $- $- variable $- flags Flags passed to script This was originally a ksh construct adopted into Bash, and unfortunately it does not seem to work reliably in Bash scripts. One possible use for it is to have a script self-test whether it is interactive. $! $! variable $! PID last job run in background PID (process id) of last job run in background $_ $_ variable $_ underscore last argument Special variable set to last argument of previous command executed. #!/bin/bash echo $_ # /bin/bash # Just called /bin/bash to run the script. du >/dev/null # So no output from command. echo $_ # du ls -al # So no output from command. echo $_ # -al (last argument) : echo $_ # : $? $? variable $? exit status exit status of a command, function, or the script itself (see ) $$ $$ variable $$ PID of script process id of script, often used in scripts to construct temp file names (see , , and ) Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified focus. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr command. This results in inconsistent command syntax and overlap of functionality, not to mention confusion. ${#string} string length parameter substitution expr length $string string length expr expr "$string" : '.*' string length expr stringZ=abcABC123ABCabc echo ${#stringZ} # 15 echo `expr length $stringZ` # 15 echo `expr "$stringZ" : '.*'` # 15 expr match "$string" '$substring' substring length expr $substring is a regular expression. expr "$string" : '$substring' substring length expr $substring is a regular expression. stringZ=abcABC123ABCabc # |------| echo `expr match "$stringZ" 'abc[A-Z]*.2'` # 8 echo `expr "$stringZ" : 'abc[A-Z]*.2'` # 8 expr index $string $substring substring index expr Numerical position in $string of first character in $substring that matches. stringZ=abcABC123ABCabc echo `expr index "$stringZ" C12` # 6 # C position. echo `expr index "$stringZ" 1c` # 3 # 'c' (in #3 position) matches before '1'. This is the near equivalent of strchr() in C. ${string:position} substring extraction Extracts substring from $string at $position. If the string parameter is * or @, then this extracts the positional parameters, This applies to either command line arguments or parameters passed to a function. starting at position. ${string:position:length} substring extraction Extracts $length characters of substring from $string at $position. stringZ=abcABC123ABCabc # 0123456789..... # 0-based indexing. echo ${stringZ:0} # abcABC123ABCabc echo ${stringZ:1} # bcABC123ABCabc echo ${stringZ:7} # 23ABCabc echo ${stringZ:7:3} # 23A # Three characters of substring. If the string parameter is * or @, then this extracts a maximum of length positional parameters, starting at position. echo ${*:2} # Echoes second and following positional parameters. echo ${@:2} # Same as above. echo ${*:2:3} # Echoes three positional parameters, starting at second. expr substr $string $position $length substring extraction expr Extracts $length characters from $string starting at $position. stringZ=abcABC123ABCabc # 123456789...... # 1-based indexing. echo `expr substr $stringZ 1 2` # ab echo `expr substr $stringZ 4 3` # ABC expr match "$string" '\($substring\)' substring extraction expr Extracts $substring at beginning of $string, where $substring is a regular expression. expr "$string" : '\($substring\)' substring extraction expr Extracts $substring at beginning of $string, where $substring is a regular expression. stringZ=abcABC123ABCabc echo `expr match "$stringZ" '\(.[b-c]*[A-Z]..[0-9]\)'` # abcABC1 echo `expr "$stringZ" : '\(.[b-c]*[A-Z]..[0-9]\)'` # abcABC1 # Both of the above forms are equivalent. ${string#substring} substring removal Strips shortest match of $substring from front of $string. ${string##substring} substring removal Strips longest match of $substring from front of $string. stringZ=abcABC123ABCabc # |----| # |----------| echo ${stringZ#a*C} # 123ABCabc # Strip out shortest match between 'a' and 'C'. echo ${stringZ##a*C} # abc # Strip out longest match between 'a' and 'C'. ${string%substring} substring removal Strips shortest match of $substring from back of $string. ${string%%substring} substring removal Strips longest match of $substring from back of $string. stringZ=abcABC123ABCabc # || # |------------| echo ${stringZ%b*c} # abcABC123ABCa # Strip out shortest match between 'b' and 'c', from back of $stringZ. echo ${stringZ%%b*c} # a # Strip out longest match between 'b' and 'c', from back of $stringZ. &cvt; ${string/substring/replacement} substring replacement Replace first match of $substring with $replacement. ${string//substring/replacement} substring replacement Replace all matches of $substring with $replacement. stringZ=abcABC123ABCabc echo ${stringZ/abc/xyz} # xyzABC123ABCabc # Replaces first match of 'abc' with 'xyz'. echo ${stringZ//abc/xyz} # xyzABC123ABCxyz # Replaces all matches of 'abc' with # 'xyz'. ${string/#substring/replacement} substring replacement If $substring matches front end of $string, substitute $replacement for $substring. ${string/%substring/replacement} substring replacement If $substring matches back end of $string, substitute $replacement for $substring. stringZ=abcABC123ABCabc echo ${stringZ/#abc/XYZ} # XYZABC123ABCabc # Replaces front-end match of 'abc' with 'xyz'. echo ${stringZ/%abc/XYZ} # abcABC123ABCXYZ # Replaces back-end match of 'abc' with 'xyz'. A Bash script may invoke the string manipulation facilities of awk as an alternative to using its built-in operations. &substringex; For more on string manipulation in scripts, refer to and the relevant section of the expr command listing. For script examples, see: ${parameter} Same as $parameter, i.e., value of the variable parameter. In certain contexts, only the less ambiguous ${parameter} form works. May be used for concatenating variables with strings. your_id=${USER}-on-${HOSTNAME} echo "$your_id" # echo "Old \$PATH = $PATH" PATH=${PATH}:/opt/bin #Add /opt/bin to $PATH for duration of script. echo "New \$PATH = $PATH" ${parameter-default} If parameter not set, use default. echo ${username-`whoami`} # Echoes the result of `whoami`, if variable $username is still unset. This is almost equivalent to ${parameter:-default}. The extra : makes a difference only when parameter has been declared, but is null. ¶msub; ${parameter=default} ${parameter:=default} If parameter not set, set it to default. Both forms nearly equivalent. The : makes a difference only when $parameter has been declared and is null, If $parameter is null in a non-interactive script, it will terminate with a 127 exit status (the Bash error code code for command not found). as above. echo ${username=`whoami`} # Variable "username" is now set to `whoami`. ${parameter+alt_value} ${parameter:+alt_value} If parameter set, use alt_value, else use null string. Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, see below. echo "###### \${parameter+alt_value} ########" echo a=${param1+xyz} echo "a = $a" # a = param2= a=${param2+xyz} echo "a = $a" # a = xyz param3=123 a=${param3+xyz} echo "a = $a" # a = xyz echo echo "###### \${parameter:+alt_value} ########" echo a=${param4:+xyz} echo "a = $a" # a = param5= a=${param5:+xyz} echo "a = $a" # a = # Different result from a=${param5+xyz} param6=123 a=${param6+xyz} echo "a = $a" # a = xyz ${parameter?err_msg} ${parameter:?err_msg} If parameter set, use it, else print err_msg. Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above. &ex6; The following expressions are the complement to the match in expr string operations (see ). These particular ones are used mostly in parsing file path names. ${#var} String length (number of characters in $var). For an array, ${#array} is the length of the first element in the array. Exceptions: ${#*} and ${#@} give the number of positional parameters. For an array, ${#array[*]} and ${#array[@]} give the number of elements in the array. &length; ${var#pattern} ${var##pattern} Remove from $var the shortest/longest part of $pattern that matches the front end of $var. A usage illustration from : # Function from "days-between.sh" example. # Strips leading zero(s) from argument passed. strip_leading_zero () # Better to strip possible leading zero(s) { # from day and/or month val=${1#0} # since otherwise Bash will interpret them return $val # as octal values (POSIX.2, sect 2.9.2.1). } Another usage illustration: echo `basename $PWD` # Basename of current working directory. echo "${PWD##*/}" # Basename of current working directory. echo echo `basename $0` # Name of script. echo $0 # Name of script. echo "${0##*/}" # Name of script. echo filename=test.data echo "${filename##*.}" # data # Extension of filename. ${var%pattern} ${var%%pattern} Remove from $var the shortest/longest part of $pattern that matches the back end of $var. Version 2 of Bash adds additional options. &pattmatching; &rfe; These constructs have been adopted from ksh. ${var:pos} Variable var expanded, starting from offset pos. ${var:pos:len} Expansion to a max of len characters of variable var, from offset pos. See for an example of the creative use of this operator. ${var/patt/replacement} First match of patt, within var replaced with replacement. If replacement is omitted, then the first match of patt is replaced by nothing, that is, deleted. ${var//patt/replacement} All matches of patt, within var replaced with replacement. As above, if replacement is omitted, then all occurrences of patt are replaced by nothing, that is, deleted. &ex7; ${var/#patt/replacement} If prefix of var matches replacement, then substitute replacement for patt. ${var/%patt/replacement} If suffix of var matches replacement, then substitute replacement for patt. &varmatch; ${!varprefix*} ${!varprefix@} Matches all previously declared variables beginning with varprefix. xyz23=whatever xyz24= a=${!xyz*} # Expands to names of declared variables beginning with "xyz". echo "a = $a" # a = xyz23 xyz24 a=${!xyz@} # Same as above. echo "a = $a" # a = xyz23 xyz24 # Bash, version 2.04, adds this feature. declare typeset command declare command typeset The declare or typeset builtins (they are exact synonyms) permit restricting the properties of variables. This is a very weak form of the typing available in certain programming languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts. -r readonly declare -r var1 (declare -r var1 works the same as readonly var1) This is the rough equivalent of the C const type qualifier. An attempt to change the value of a readonly variable fails with an error message. -i integer declare -i number # The script will treat subsequent occurrences of "number" as an integer. number=3 echo "number = $number" # number = 3 number=three echo "number = $number" # number = 0 # Tries to evaluate "three" as an integer. Note that certain arithmetic operations are permitted for declared integer variables without the need for expr or let. -a array declare -a indices The variable indices will be treated as an array. -f functions declare -f A declare -f line with no arguments in a script causes a listing of all the functions previously defined in that script. declare -f function_name A declare -f function_name in a script lists just the function named. -x export declare -x var3 This declares a variable as available for exporting outside the environment of the script itself. var=$value declare -x var3=373 The declare command permits assigning a value to a variable in the same statement as setting its properties. &ex20; Assume that the value of a variable is the name of a second variable. Is it somehow possible to retrieve the value of this second variable from the first one? For example, if a=letter_of_alphabet and letter_of_alphabet=z, can a reference to a return z? This can indeed be done, and it is called an indirect reference. It uses the unusual eval var1=\$$var2 notation. &indref; &coltotaler2; This method of indirect referencing is a bit tricky. If the second order variable changes its value, then the the first order variable must be properly dereferenced (as in the above example). Fortunately, the ${!variable} notation introduced with version 2 of Bash (see ) makes indirect referencing more intuitive. $RANDOM variable $RANDOM $RANDOM is an internal Bash function (not a constant) that returns a pseudorandom integer in the range 0 - 32767. $RANDOM should not be used to generate an encryption key. &ex21; Just how random is RANDOM? The best way to test this is to write a script that tracks the distribution of random numbers generated by RANDOM. Let's roll a RANDOM die a few times... &randomtest; As we have seen in the last example, it is best to reseed the RANDOM generator each time it is invoked. Using the same seed for RANDOM repeats the same series of numbers. (This mirrors the behavior of the random() function in C.) &seedingrandom; The /dev/urandom device-file provides a means of generating much more random pseudorandom numbers than the $RANDOM variable. dd if=/dev/urandom of=targetfile bs=1 count=XX creates a file of well-scattered pseudorandom numbers. However, assigning these numbers to a variable in a script requires a workaround, such as filtering through od (as in above example) or using dd (see ). There are also other means of generating pseudorandom numbers in a script. Awk provides a convenient means of doing this. &random2; Similar to the let command, the ((...)) construct permits arithmetic expansion and evaluation. In its simplest form, a=$(( 5 + 3 )) would set a to 5 + 3, or 8. However, this double parentheses construct is also a mechanism for allowing C-type manipulation of variables in Bash. &cvars; See also . Operations on code blocks are the key to structured, organized shell scripts. Looping and branching constructs provide the tools for accomplishing this. A loop is a block of code that iterates (repeats) a list of commands as long as the loop control condition is true. for (in) for in do done loop for This is the basic looping construct. It differs significantly from its C counterpart. for arg in list do  command done During each pass through the loop, arg takes on the value of each variable in the list. for arg in "$var1" "$var2" "$var3" ... "$varN" # In pass 1 of the loop, $arg = $var1 # In pass 2 of the loop, $arg = $var2 # In pass 3 of the loop, $arg = $var3 # ... # In pass N of the loop, $arg = $varN # Arguments in [list] quoted to prevent possible word splitting. The argument list may contain wild cards. If do is on same line as for, there needs to be a semicolon after list. for arg in list ; do &ex22; Each [list] element may contain multiple parameters. This is useful when processing parameters in groups. In such cases, use the set command (see ) to force parsing of each [list] element and assignment of each component to the positional parameters. &ex22a; A variable may supply the [list] in a for loop. &fileinfo; The [list] in a for loop may contain filename globbing, that is, using wildcards for filename expansion. &listglob; Omitting the in [list] part of a for loop causes the loop to operate on $@, the list of arguments given on the command line to the script. A particularly clever illustration of this is . &ex23; It is possible to use command substitution to generate the [list] in a for loop. See also , and . &forloopcmd; This is a somewhat more complex example of using command substitution to create the [list]. &bingrep; More of the same. &userlist; A final example of the [list] resulting from command substitution. &findstring; The output of a for loop may be piped to a command or commands. &symlinks; The stdout of a loop may be redirected to a file, as this slight modification to the previous example shows. &symlinks2; There is an alternative syntax to a for loop that will look very familiar to C programmers. This requires double parentheses. &forloopc; See also , , and . --- Now, a for-loop used in a real-life context. &ex24; while while do done loop while This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the number of loop repetitions is not known beforehand. while condition do  command done As is the case with for/in loops, placing the do on the same line as the condition test requires a semicolon. while condition ; do Note that certain specialized while loops, as, for example, a getopts construct, deviate somewhat from the standard template given here. &ex25; &ex26; A while loop may have multiple conditions. Only the final condition determines when the loop terminates. This necessitates a slightly different loop syntax, however. &ex26a; As with a for loop, a while loop may employ C-like syntax by using the double parentheses construct (see also ). &whloopc; A while loop may have its stdin redirected to a file by a < at its end. until until do done loop until This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is false (opposite of while loop). until condition-is-true do  command done Note that an until loop tests for the terminating condition at the top of the loop, differing from a similar construct in some programming languages. As is the case with for/in loops, placing the do on the same line as the condition test requires a semicolon. until condition-is-true ; do &ex27; A nested loop is a loop within a loop, an inner loop within the body of an outer one. What happens is that the first pass of the outer loop triggers the inner loop, which executes to completion. Then the second pass of the outer loop triggers the inner loop again. This repeats until the outer loop finishes. Of course, a break within either the inner or outer loop may interrupt this process. &nestedloop; See for an illustration of nested while loops, and to see a while loop nested inside an until loop. break continue loop break loop continue break continue The break and continue loop control commands These are shell builtins, whereas other loop commands, such as while and case, are keywords. correspond exactly to their counterparts in other programming languages. The break command terminates the loop (breaks out of it), while continue causes a jump to the next iteration of the loop, skipping all the remaining commands in that particular loop cycle. &ex28; The break command may optionally take a parameter. A plain break terminates only the innermost loop in which it is embedded, but a break N breaks out of N levels of loop. &breaklevels; The continue command, similar to break, optionally takes a parameter. A plain continue cuts short the current iteration within its loop and begins the next. A continue N terminates all remaining iterations at its loop level and continues with the next iteration at the loop N levels above. &continuelevels; The continue N construct is difficult to understand and tricky to use in any meaningful context. It is probably best avoided. The case and select constructs are technically not loops, since they do not iterate the execution of a code block. Like loops, however, they direct program flow according to conditions at the top or bottom of the block. case (in) / esac case in esac switch ;; menus The case construct is the shell equivalent of switch in C/C++. It permits branching to one of a number of code blocks, depending on condition tests. It serves as a kind of shorthand for multiple if/then/else statements and is an appropriate tool for creating menus. case "$variable" in  "$condition1" )  command  ;;  "$condition2" )  command  ;; esac Quoting the variables is not mandatory, since word splitting does not take place. Each test line ends with a right paren ). Each condition block ends with a double semicolon ;;. The entire case block terminates with an esac (case spelled backwards). &ex29; &ex30; An exceptionally clever use of case involves testing for command-line parameters. #! /bin/bash case "$1" in "") echo "Usage: ${0##*/} <filename>"; exit 65;; # No command-line parameters, # or first parameter empty. # Note that ${0##*/} is ${var##pattern} param substitution. Net result is $0. -*) FILENAME=./$1;; # If filename passed as argument ($1) starts with a dash, # replace it with ./$1 # so further commands don't interpret it as an option. * ) FILENAME=$1;; # Otherwise, $1. esac &casecmd; A case construct can filter strings for globbing patterns. &matchstring; &isalpha; select select menus The select construct, adopted from the Korn Shell, is yet another tool for building menus. select variable in list do  command  break done This prompts the user to enter one of the choices presented in the variable list. Note that select uses the PS3 prompt (#? ) by default, but that this may be changed. &ex31; If in list is omitted, then select uses the list of command line arguments ($@) passed to the script or to the function in which the select construct is embedded. Compare this to the behavior of a for variable in list construct with the in list omitted. &ex32; See also . builtin A builtin is a command contained within the Bash tool set, literally built in. A builtin may be a synonym to a system command of the same name, but Bash reimplements it internally. This is either for performance reasons (builtins execute much faster than external commands, which usually require forking off a process) or because a particular builtin needs direct access to the shell internals. For example, the Bash echo command is not the same as /bin/echo, although their behavior is almost identical. A keyword is a reserved word, token or operator. Keywords have a special meaning to the shell, and indeed are the building blocks of the shell's syntax. As examples, for, while, do, and ! are keywords. Similar to a builtin, a keyword is hard-coded into Bash, but unlike a builtin, a keyword is not by itself a command, but part of a larger command structure. An exception to this is the time command, listed in the official Bash documentation as a keyword. echo echo command echo prints (to stdout) an expression or variable (see ). echo Hello echo $a An echo requires the -e option to print escaped characters. See . Normally, each echo command prints a terminal newline, but the -n option suppresses this. An echo can be used to feed a sequence of commands down a pipe. if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]] then echo "$VAR contains the substring sequence \"txt\"" fi An echo, in combination with command substitution can set a variable. a=`echo "HELLO" | tr A-Z a-z` See also , , , and . Be aware that echo `command` deletes any linefeeds that the output of command generates. The $IFS (internal field separator) variable normally contains \n (linefeed) as one of its set of whitespace characters. Bash therefore splits the output of command at linefeeds into arguments to echo. Then echo outputs these arguments, separated by spaces. bash$ ls -l /usr/share/apps/kjezz/sounds -rw-r--r-- 1 root root 1407 Nov 7 2000 reflect.au -rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au bash$ echo `ls -l /usr/share/apps/kjezz/sounds` total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au This command is a shell builtin, and not the same as /bin/echo, although its behavior is similar. bash$ type -a echo echo is a shell builtin echo is /bin/echo printf printf command printf The printf, formatted print, command is an enhanced echo. It is a limited variant of the C language printf, and the syntax is somewhat different. printf format-string parameter This is the Bash builtin version of the /bin/printf or /usr/bin/printf command. See the printf manpage (of the system command) for in-depth coverage. Older versions of Bash may not support printf. &ex47; Formatting error messages is a useful application of printf E_BADDIR=65 var=nonexistent_directory error() { printf "$@" >&2 # Formats positional params passed, and sents them to stderr. echo exit $E_BADDIR } cd $var || error $"Can't cd to %s." "$var" # Thanks, S.C. read read command read Reads the value of a variable from stdin, that is, interactively fetches input from the keyboard. The -a option lets read get array variables (see ). &ex36; Normally, inputting a \ suppresses a newline during input to a read. The -r option causes an inputted \ to be interpreted literally. &readr; The read command has some interesting options that permit echoing a prompt and even reading keystrokes without hitting ENTER. # Read a keypress without hitting ENTER. read -s -n1 -p "Hit a key " keypress echo; echo "Keypress was "\"$keypress\""." # -s option means do not echo input. # -n N option means accept only N characters of input. # -p option means echo the following prompt before reading input. # Using these options is tricky, since they need to be in the correct order. The -t option to read permits timed input (see ). The read command may also read its variable value from a file redirected to stdin. If the file contains more than one line, only the first line is assigned to the variable. If read has more than one parameter, then each of these variables gets assigned a successive whitespace-delineated string. Caution! &readredir; cd cd command cd The familiar cd change directory command finds use in scripts where execution of a command requires being in a specified directory. (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -) [from the previously cited example by Alan Cox] The -P (physical) option to cd causes it to ignore symbolic links. cd - changes to $OLDPWD, the previous working directory. pwd pwd command pwd $PWD variable $PWD directory working Print Working Directory. This gives the user's (or script's) current directory (see ). The effect is identical to reading the value of the builtin variable $PWD. pushd popd dirs pushd command pushd popd command popd dirs command dirs directory working bookmark This command set is a mechanism for bookmarking working directories, a means of moving back and forth through directories in an orderly manner. A pushdown stack is used to keep track of directory names. Options allow various manipulations of the directory stack. pushd dir-name pushes the path dir-name onto the directory stack and simultaneously changes the current working directory to dir-name popd removes (pops) the top directory path name off the directory stack and simultaneously changes the current working directory to that directory popped from the stack. dirs lists the contents of the directory stack (compare this with the $DIRSTACK variable). A successful pushd or popd will automatically invoke dirs. Scripts that require various changes to the current working directory without hard-coding the directory name changes can make good use of these commands. Note that the implicit $DIRSTACK array variable, accessible from within a script, holds the contents of the directory stack. &ex37; let let command let The let command carries out arithmetic operations on variables. In many cases, it functions as a less complex version of expr. &ex46; eval eval command eval eval arg1 [arg2] ... [argN] Translates into commands the arguments in a list (useful for code generation within a script). &ex43; &ex44; &rot14; The eval command can be risky, and normally should be avoided when there exists a reasonable alternative. An eval $COMMANDS executes the contents of COMMANDS, which may contain such unpleasant surprises as rm -rf *. Running an eval on unfamiliar code written by persons unknown is living dangerously. set set command set The set command changes the value of internal script variables. One use for this is to toggle option flags which help determine the behavior of the script. Another application for it is to reset the positional parameters that a script sees as the result of a command (set `command`). The script can then parse the fields of the command output. &ex34; See also . unset unset command unset The unset command deletes a shell variable, effectively setting it to null. Note that this command does not affect positional parameters. bash$ unset PATH bash$ echo $PATH bash$ &uns; export export command export The export command makes available variables to all child processes of the running script or shell. Unfortunately, there is no way to export variables back to the parent process, to the process that called or invoked the script or shell. One important use of export command is in startup files, to initialize and make accessible environmental variables to subsequent user processes. &coltotaler3; It is possible to initialize and export variables in the same operation, as in export var1=xxx. declare typeset declare command declare typeset command typeset The declare and typeset commands specify and/or restrict properties of variables. readonly readonly command readonly Same as declare -r, sets a variable as read-only, or, in effect, as a constant. Attempts to change the variable fail with an error message. This is the shell analog of the C language const type qualifier. getopts getopts command getopts $OPTIND variable $OPTIND $OPTARG variable $OPTARG This powerful tool parses command line arguments passed to the script. This is the bash analog of the getopt library function familiar to C programmers. It permits passing and concatenating multiple options A option is an argument that acts as a flag, switching script behaviors on or off. The argument associated with a particular option indicates the behavior that the option (flag) switches on or off. and associated arguments to a script (for example scriptname -abc -e /usr/local). The getopts construct uses two implicit variables. $OPTIND is the argument pointer (OPTion INDex) and $OPTARG (OPTion ARGument) the (optional) argument attached to an option. A colon following the option name in the declaration tags that option as having an associated argument. A getopts construct usually comes packaged in a while loop, which processes the options and arguments one at a time, then decrements the implicit $OPTIND variable to step to the next. The arguments must be passed from the command line to the script preceded by a minus (-) or a plus (+). It is the prefixed - or + that lets getopts recognize command-line arguments as options. In fact, getopts will not process arguments without the prefixed - or +, and will terminate option processing at the first argument encountered lacking them. The getopts template differs slightly from the standard while loop, in that it lacks condition brackets. The getopts construct replaces the obsolete getopt command. while getopts ":abcde:fg" Option # Initial declaration. # a, b, c, d, e, f, and g are the options (flags) expected. # The : after option 'e' shows it will have an argument passed with it. do case $Option in a ) # Do something with variable 'a'. b ) # Do something with variable 'b'. ... e) # Do something with 'e', and also with $OPTARG, # which is the associated argument passed with option 'e'. ... g ) # Do something with variable 'g'. esac done shift $(($OPTIND - 1)) # Move argument pointer to next. # All this is not nearly as complicated as it looks <grin>. &ex33; source . (dot command) source command source . command . This command, when invoked from the command line, executes a script. Within a script, a source file-name loads the file file-name. This is the shell scripting equivalent of a C/C++ #include directive. It is useful in situations when multiple scripts use a common data file or function library. &ex38; File data-file for , above. Must be present in same directory. &ex38bis; exit exit command exit Unconditionally terminates a script. The exit command may optionally take an integer argument, which is returned to the shell as the exit status of the script. It is a good practice to end all but the simplest scripts with an exit 0, indicating a successful run. If a script terminates with an exit lacking an argument, the exit status of the script is the exit status of the last command executed in the script, not counting the exit. exec exec command exec This shell builtin replaces the current process with a specified command. Normally, when the shell encounters a command, it forks off When a command or the shell itself initiates (or spawns) a new subprocess to carry out a task, this is called forking. This new process is the child, and the process that forked it off is the parent. While the child process is doing its work, the parent process is still running. a child process to actually execute the command. Using the exec builtin, the shell does not fork, and the command exec'ed replaces the shell. When used in a script, therefore, it forces an exit from the script when the exec'ed command terminates. For this reason, if an exec appears in a script, it would probably be the final command. An exec also serves to reassign file descriptors. exec <zzz-file replaces stdin with the file zzz-file (see ). &ex54; The -exec option to find is not the same as the exec shell builtin. shopt shopt command shopt This command permits changing shell options on the fly (see and ). It often appears in the Bash startup files, but also has its uses in scripts. Needs version 2 or later of Bash. shopt -s cdspell # Allows minor misspelling directory names with 'cd' command. true true command true A command that returns a successful (zero) exit status, but does nothing else. # Endless loop while true # alias for ":" do operation-1 operation-2 ... operation-n # Need a way to break out of loop. done false false command false A command that returns an unsuccessful exit status, but does nothing else. # Null loop while false do # The following code will not execute. operation-1 operation-2 ... operation-n # Nothing happens! done type [cmd] type command type variable which Similar to the which external command, type cmd gives the full pathname to cmd. Unlike which, type is a Bash builtin. The useful -a option to type accesses identifies keywords and builtins, and also locates system commands with identical names. bash$ type '[' [ is a shell builtin bash$ type -a '[' [ is a shell builtin [ is /usr/bin/[ hash [cmds] hash command hash $PATH variable $PATH Record the path name of specified commands (in the shell hash table), so the shell or script will not need to search the $PATH on subsequent calls to those commands. When hash is called with no arguments, it simply lists the commands that have been hashed. The -r option resets the hash table. help help command help COMMAND looks up a short usage summary of the shell builtin COMMAND. This is the counterpart to whatis, but for builtins. bash$ help exit exit: exit [n] Exit the shell with a status of N. If N is omitted, the exit status is that of the last command executed. Certain of the following job control commands take a job identifier as an argument. See the table at end of the chapter. jobs jobs command jobs ps command ps Lists the jobs running in the background, giving the job number. Not as useful as ps. It is all too easy to confuse jobs and processes. Certain builtins, such as kill, disown, and wait accept either a job number or a process number as an argument. The fg, bg and jobs commands accept only a job number. bash$ sleep 100 & [1] 1384 bash $ jobs [1]+ Running sleep 100 & 1 is the job number (jobs are maintained by the current shell), and 1384 is the process number (processes are maintained by the system). To kill this job/process, either a kill %1 or a kill 1384 works. Thanks, S.C. disown disown command disown Remove job(s) from the shell's table of active jobs. fg bg fg command foreground background command bg The fg command switches a job running in the background into the foreground. The bg command restarts a suspended job, and runs it in the background. If no job number is specified, then the fg or bg command acts upon the currently running job. wait wait command wait Stop script execution until all jobs running in background have terminated, or until the job number or process id specified as an option terminates. Returns the exit status of waited-for command. You may use the wait command to prevent a script from exiting before a background job finishes executing (this would create a dreaded orphan process). &ex39; Optionally, wait can take a job identifier as an argument, for example, wait%1 or wait $PPID. See the job id table. Within a script, running a command in the background with an ampersand (&) may cause the script to hang until ENTER is hit. This seems to occur with commands that write to stdout. It can be a major annoyance. #!/bin/bash # test.sh ls -l & echo "Done." bash$ ./test.sh Done. [bozo@localhost test-scripts]$ total 1 -rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh _ Placing a wait after the background command seems to remedy this. #!/bin/bash # test.sh ls -l & echo "Done." wait bash$ ./test.sh Done. [bozo@localhost test-scripts]$ total 1 -rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh Redirecting the output of the command to a file or even to /dev/null also takes care of this problem. suspend suspend command suspend This has a similar effect to ControlZ, but it suspends the shell (the shell's parent process should resume it at an appropriate time). logout logout command log out Exit a login shell, optionally specifying an exit status. times times command times Gives statistics on the system time used in executing commands, in the following form: 0m0.020s 0m0.020s This capability is of very limited value, since it is uncommon to profile and benchmark shell scripts. kill kill command kill Forcibly terminate a process by sending it an appropriate terminate signal (see ). kill -l lists all the signals. A kill -9 is a sure kill, which will usually terminate a process that stubbornly refuses to die with a plain kill. Sometimes, a kill -15 works. A zombie process, that is, a process whose parent has terminated, cannot be killed (you can't kill something that is already dead), but init will usually clean it up sooner or later. command command command command The command COMMAND directive disables aliases and functions for the command COMMAND. This is one of three shell directives that effect script command processing. The others are builtin and enable. builtin builtin command builtin Invoking builtin BUILTIN_COMMAND runs the command BUILTIN_COMMAND as a shell builtin, temporarily disabling both functions and external system commands with the same name. enable enable command enable This either enables or disables a shell builtin command. As an example, enable -n kill disables the shell builtin kill, so that when Bash subsequently encounters kill, it invokes /bin/kill. The -a option to enable lists all the shell builtins, indicating whether or not they are enabled. The -f filename option lets enable load a builtin as a shared library (DLL) module from a properly compiled object file. The C source for a number of loadable builtins is typically found in the /usr/share/doc/bash-?.??/functions directory. Note that the -f option to enable is not portable to all systems. . autoload autoload command autoloader This is a port to Bash of the ksh autoloader. With autoload in place, a function with an autoload declaration will load from an external file at its first invocation. The same effect as autoload can be achieved with typeset -fu. This saves system resources. Note that autoload is not a part of the core Bash installation. It needs to be loaded in with enable -f (see above). Notation Meaning %N Job number [N] %S Invocation (command line) of job begins with string S %?S Invocation (command line) of job contains within it string S %% current job (last job stopped in foreground or started in background) %+ current job (last job stopped in foreground or started in background) %- Last job $! Last background process Standard UNIX commands make shell scripts more versatile. The power of scripts comes from coupling system commands and shell directives with simple programming constructs. ls ls command ls The basic file list command. It is all too easy to underestimate the power of this humble command. For example, using the -R, recursive option, ls provides a tree-like listing of a directory structure. Other interesting options are -S, sort listing by file size, -t, sort by file modification time, and -i, show file inodes (see ). &ex40; cat tac cat command cat tac command tac cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it is commonly used to concatenate files. cat filename cat file.1 file.2 file.3 > file.123 The -n option to cat inserts consecutive numbers before all lines of the target file(s). The -b option numbers only the non-blank lines. The -v option echoes nonprintable characters, using ^ notation. The -s option squeezes multiple consecutive blank lines into a single blank line. See also and . tac, is the inverse of cat, listing a file backwards from its end. rev rev command rev reverses each line of a file, and outputs to stdout. This is not the same effect as tac, as it preserves the order of the lines, but flips each one around. bash$ cat file1.txt This is line 1. This is line 2. bash$ tac file1.txt This is line 2. This is line 1. bash$ rev file1.txt .1 enil si sihT .2 enil si sihT cp cp command cp This is the file copy command. cp file1 file2 copies file1 to file2, overwriting file2 if it already exists (see ). Particularly useful are the -a archive flag (for copying an entire directory tree) and the -r and -R recursive flags. mv This is the file move command. It is equivalent to a combination of cp and rm. It may be used to move multiple files to a directory, or even to rename a directory. For some examples of using mv in a script, see and . When used in a non-interactive script, mv takes the -f (force) option to bypass user input. rm rm command rm Delete (remove) a file or files. The -f option forces removal of even readonly files, and is useful for bypassing user input in a script. When used with the recursive flag -r, this command removes files all the way down the directory tree. rmdir rmdir command rmdir Remove directory. The directory must be empty of all files, including invisible dotfiles, These are files whose names begin with a dot, such as ~/.Xdefaults. Such filenames do not show up in a normal ls listing, and they cannot be deleted by an accidental rm -rf *. Dotfiles are generally used as setup and configuration files in a user's home directory. for this command to succeed. mkdir mkdir command mkdir Make directory, creates a new directory. mkdir -p project/programs/December creates the named directory. The -p option automatically creates any necessary parent directories. chmod chmod command chmod Changes the attributes of an existing file (see ). chmod +x filename # Makes "filename" executable for all users. chmod u+s filename # Sets "suid" bit on "filename" permissions. # An ordinary user may execute "filename" with same privileges as the file's owner. # (This does not apply to shell scripts.) chmod 644 filename # Makes "filename" readable/writable to owner, readable to # others # (octal mode). chmod 1777 directory-name # Gives everyone read, write, and execute permission in directory, # however also sets the "sticky bit". # This means that only the owner of the directory, # owner of the file, and, of course, root # can delete any particular file in that directory. chattr chattr command chattr Change file attributes. This has the same effect as chmod above, but with a different invocation syntax, and it works only on an ext2 filesystem. ln Creates links to pre-existings files. Most often used with the -s, symbolic or soft link flag. This permits referencing the linked file by more than one name and is a superior alternative to aliasing (see ). ln -s oldfile newfile links the previously existing oldfile to the newly created link, newfile. find find command find {} special character {} \; escaped character \; -exec COMMAND \; Carries out COMMAND on each file that find scores a hit on. COMMAND terminates with \; (the ; is escaped to make certain the shell passes it to find literally, which concludes the command sequence). If COMMAND contains {}, then find substitutes the full path name of the selected file. bash$ find ~/ -name '*.txt' /home/bozo/.kde/share/apps/karm/karmdata.txt /home/bozo/misc/irmeyc.txt /home/bozo/test-scripts/1.txt find /home/bozo/projects -mtime 1 # Lists all files in /home/bozo/projects directory tree # that were modified within the last day. find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {} \; # Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files. # There a few extraneous hits - how can they be filtered out? # Perhaps by: find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \ | grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$' # [:digit:] is one of the character classes # introduced with the POSIX 1003.2 standard. # Thanks, S.C. The -exec option to find should not be confused with the exec shell builtin. &ex57; &idelete; See , , and for scripts using find. Its manpage provides more detail on this complex and powerful command. xargs xargs command xargs A filter for feeding arguments to a command, and also a tool for assembling the commands themselves. It breaks a data stream into small enough chunks for filters and commands to process. Consider it as a powerful replacement for backquotes. In situations where backquotes fail with a too many arguments error, substituting xargs often works. Normally, xargs reads from stdin or from a pipe, but it can also be given the output of a file. The default command for xargs is echo. This means that input piped to xargs may have linefeeds and other whitespace characters stripped out. bash$ ls -l total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2 bash$ ls -l | xargs total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2 ls | xargs -p -l gzip gzips every file in current directory, one at a time, prompting before each operation. An interesting xargs option is -n NN, which limits to NN the number of arguments passed. ls | xargs -n 8 echo lists the files in the current directory in 8 columns. Another useful option is -0, in combination with find -print0 or grep -lZ. This allows handling arguments containing whitespace or quotes. find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f grep -rliwZ GUI / | xargs -0 rm -f Either of the above will remove any file containing GUI. (Thanks, S.C.) &ex41; &ex42; expr expr command expr All-purpose expression evaluator: Concatenates and evaluates the arguments according to the operation given (arguments must be separated by spaces). Operations may be arithmetic, comparison, string, or logical. expr 3 + 5 returns 8 expr 5 % 3 returns 2 expr 5 \* 3 returns 15 The multiplication operator must be escaped when used in an arithmetic expression with expr. y=`expr $y + 1` Increment a variable, with the same effect as let y=y+1 and y=$(($y+1)) This is an example of arithmetic expansion. z=`expr substr $string $position $length` Extract substring of $length characters, starting at $position. &ex45; The : operator can substitute for match. For example, b=`expr $a : [0-9]*` is the exact equivalent of b=`expr match $a [0-9]*` in the above listing. &ex45a; Perl and sed have far superior string parsing facilities. A short Perl or sed subroutine within a script (see ) is an attractive alternative to using expr. See for more on string operations. date date command date Simply invoked, date prints the date and time to stdout. Where this command gets interesting is in its formatting and parsing options. &ex51; The -u option gives the UTC (Universal Coordinated Time). bash$ date Fri Mar 29 21:07:39 MST 2002 bash$ date -u Sat Mar 30 04:07:42 UTC 2002 zdump zdump command time zone dump Echoes the time in a specified time zone. bash$ zdump EST EST Tue Sep 18 22:09:22 2001 EST time time command time Outputs very verbose timing statistics for executing a command. time ls -l / gives something like this: 0.00user 0.01system 0:00.05elapsed 16%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (149major+27minor)pagefaults 0swaps See also the very similar times command in the previous section. As of version 2.0 of Bash, time became a shell reserved word, with slightly altered behavior in a pipeline. touch touch command touch Utility for updating access/modification times of a file to current system time or other specified time, but also useful for creating a new file. The command touch zzz will create a new file of zero length, named zzz, assuming that zzz did not previously exist. Time-stamping empty files in this way is useful for storing date information, for example in keeping track of modification times on a project. The touch command is equivalent to : >> newfile (for ordinary files). at at command at crond command crond The at job control command executes a given set of commands at a specified time. Superficially, it resembles crond, however, at is chiefly useful for one-time execution of a command set. at 2pm January 15 prompts for a set of commands to execute at that time. These commands should be shell-script compatible, since, for all practical purposes, the user is typing in an executable shell script a line at a time. Input terminates with a Ctl-D. Using either the -f option or input redirection (<), at reads a command list from a file. This file is an executable shell script, though it should, of course, be noninteractive. Particularly clever is including the run-parts command in the file to execute a different set of scripts. bash$ at 2:30 am Friday < at-jobs.list job 2 at 2000-10-27 02:30 batch batch command batch at command at The batch job control command is similar to at, but it runs a command list when the system load drops below .8. Like at, it can read commands from a file with the -f option. cal cal command cal Prints a neatly formatted monthly calendar to stdout. Will do current year or a large range of past and future years. sleep sleep command sleep This is the shell equivalent of a wait loop. It pauses for a specified number of seconds, doing nothing. This can be useful for timing or in processes running in the background, checking for a specific event every so often (see ). sleep 3 # Pauses 3 seconds. The sleep command defaults to seconds, but minute, hours, or days may also be specified. sleep 3 h # Pauses 3 hours! usleep usleep command usleep Microsleep (the u may be read as the Greek mu, or micro prefix). This is the same as sleep, above, but sleeps in microsecond intervals. This can be used for fine-grain timing, or for polling an ongoing process at very frequent intervals. usleep 30 # Pauses 30 microseconds. The usleep command does not provide particularly accurate timing, and is therefore unsuitable for critical timing loops. hwclock clock hwclock command hwclock clock command clock The hwclock command accesses or adjusts the machine's hardware clock. Some options require root privileges. The /etc/rc.d/rc.sysinit startup file uses hwclock to set the system time from the hardware clock at bootup. The clock command is a synonym for hwclock. sort sort command sort File sorter, often used as a filter in a pipe. This command sorts a text stream or file forwards or backwards, or according to various keys or character positions. Using the -m option, it merges presorted input files. The info page lists its many capabilities and options. See and . tsort tsort command topological sort Topological sort, reading in pairs of whitespace-separated strings and sorting according to input patterns. diff patch diff command diff patch command patch diff: flexible file comparison utility. It compares the target files line-by-line sequentially. In some applications, such as comparing word dictionaries, it may be helpful to filter the files through sort and uniq before piping them to diff. diff file-1 file-2 outputs the lines in the files that differ, with carets showing which file each particular line belongs to. The --side-by-side option to diff outputs each compared file, line by line, in separate columns, with non-matching lines marked. There are available various fancy frontends for diff, such as spiff, wdiff, xdiff, and mgdiff. The diff command returns an exit status of 0 if the compared files are identical, and 1 if they differ. This permits use of diff in a test construct within a shell script (see below). A common use for diff is generating difference files to be used with patch The -e option outputs files suitable for ed or ex scripts. patch: flexible versioning utility. Given a difference file generated by diff, patch can upgrade a previous version of a package to a newer version. It is much more convenient to distribute a relatively small diff file than the entire body of a newly revised package. Kernel patches have become the preferred method of distributing the frequent releases of the Linux kernel. patch -p1 <patch-file # Takes all the changes listed in 'patch-file' # and applies them to the files referenced therein. # This upgrades to a newer version of the package. Patching the kernel: cd /usr/src gzip -cd patchXX.gz | patch -p0 # Upgrading kernel source using 'patch'. # From the Linux kernel docs "README", # by anonymous author (Alan Cox?). The diff command can also recursively compare directories (for the filenames present). bash$ diff -r ~/notes1 ~/notes2 Only in /home/bozo/notes1: file02 Only in /home/bozo/notes1: file03 Only in /home/bozo/notes2: file04 Use zdiff to compare gzipped files. diff3 diff3 command diff3 An extended version of diff that compares three files at a time. This command returns an exit value of 0 upon successful execution, but unfortunately this gives no information about the results of the comparison. bash$ diff3 file-1 file-2 file-3 ==== 1:1c This is line 1 of "file-1". 2:1c This is line 1 of "file-2". 3:1c This is line 1 of "file-3" sdiff sdiff command sdiff Compare and/or edit two files in order to merge them into an output file. Because of its interactive nature, this command would find little use in a script. cmp cmp command cmp The cmp command is a simpler version of diff, above. Whereas diff reports the differences between two files, cmp merely shows at what point they differ. Like diff, cmp returns an exit status of 0 if the compared files are identical, and 1 if they differ. This permits use in a test construct within a shell script. &filecomp; Use zcmp on gzipped files. comm comm command comm Versatile file comparison utility. The files must be sorted for this to be useful. comm -options first-file second-file comm file-1 file-2 outputs three columns: column 1 = lines unique to file-1 column 2 = lines unique to file-2 column 3 = lines common to both. The options allow suppressing output of one or more columns. -1 suppresses column 1 -2 suppresses column 2 -3 suppresses column 3 -12 suppresses both columns 1 and 2, etc. uniq uniq command uniq This filter removes duplicate lines from a sorted file. It is often seen in a pipe coupled with sort. cat list-1 list-2 list-3 | sort | uniq > final.list # Concatenates the list files, # sorts them, # removes duplicate lines, # and finally writes the result to an output file. The useful -c option prefixes each line of the input file with its number of occurrences. bash$ cat testfile This line occurs only once. This line occurs twice. This line occurs twice. This line occurs three times. This line occurs three times. This line occurs three times. bash$ uniq -c testfile 1 This line occurs only once. 2 This line occurs twice. 3 This line occurs three times. bash$ sort testfile | uniq -c | sort -nr 3 This line occurs three times. 2 This line occurs twice. 1 This line occurs only once. The sort INPUTFILE | uniq -c | sort -nr command string produces a frequency of occurrence listing on the INPUTFILE file (the -nr options to sort cause a reverse numerical sort). This template finds use in analysis of log files and dictionary lists, and wherever the lexical structure of a document needs to be examined. &wf; bash$ cat testfile This line occurs only once. This line occurs twice. This line occurs twice. This line occurs three times. This line occurs three times. This line occurs three times. bash$ ./wf.sh testfile 6 this 6 occurs 6 line 3 times 3 three 2 twice 1 only 1 once expand unexpand expand command expand unexpand command unexpand The expand filter converts tabs to spaces. It is often used in a pipe. The unexpand filter converts spaces to tabs. This reverses the effect of expand. cut cut command cut awk command awk A tool for extracting fields from files. It is similar to the print $N command set in awk, but more limited. It may be simpler to use cut in a script than awk. Particularly important are the -d (delimiter) and -f (field specifier) options. Using cut to obtain a listing of the mounted filesystems: cat /etc/mtab | cut -d ' ' -f1,2 Using cut to list the OS and kernel version: uname -a | cut -d" " -f1,3,11,12 Using cut to extract message headers from an e-mail folder: bash$ grep '^Subject:' read-messages | cut -c10-80 Re: Linux suitable for mission-critical apps? MAKE MILLIONS WORKING AT HOME!!! Spam complaint Re: Spam complaint Using cut to parse a file: # List all the users in /etc/passwd. FILENAME=/etc/passwd for user in $(cut -d: -f1 $FILENAME) do echo $user done # Thanks, Oleg Philon for suggesting this. cut -d ' ' -f2,3 filename is equivalent to awk -F'[ ]' '{ print $2, $3 }' filename See also . paste paste command paste cut command cut Tool for merging together different files into a single, multi-column file. In combination with cut, useful for creating system log files. join join command join Consider this a special-purpose cousin of paste. This powerful utility allows merging two files in a meaningful fashion, which essentially creates a simple version of a relational database. The join command operates on exactly two files, but pastes together only those lines with a common tagged field (usually a numerical label), and writes the result to stdout. The files to be joined should be sorted according to the tagged field for the matchups to work properly. File: 1.data 100 Shoes 200 Laces 300 Socks File: 2.data 100 $40.00 200 $1.00 300 $2.00 bash$ join 1.data 2.data File: 1.data 2.data 100 Shoes $40.00 200 Laces $1.00 300 Socks $2.00 The tagged field appears only once in the output. head head command head lists the beginning of a file to stdout (the default is 10 lines, but this can be changed). It has a number of interesting options. &rnd; See also . tail tail command tail lists the end of a file to stdout (the default is 10 lines). Commonly used to keep track of changes to a system logfile, using the -f option, which outputs lines appended to the file. &ex12; See also , and . grep grep command grep A multi-purpose file search tool that uses regular expressions. It was originally a command/filter in the venerable ed line editor, g/re/p, that is, global - regular expression - print. grep pattern file Search the target file(s) for occurrences of pattern, where pattern may be literal text or a regular expression. bash$ grep '[rst]ystem.$' osinfo.txt The GPL governs the distribution of the Linux operating system. If no target file(s) specified, grep works as a filter on stdout, as in a pipe. bash$ ps ax | grep clock 765 tty1 S 0:00 xclock 901 pts/1 S 0:00 grep clock The -i option causes a case-insensitive search. The -w option matches only whole words. The -l option lists only the files in which matches were found, but not the matching lines. The -r (recursive) option searches files in the current working directory and all subdirectories below it. The -n option lists the matching lines, together with line numbers. bash$ grep -n Linux osinfo.txt 2:This is a file containing information about Linux. 6:The GPL governs the distribution of the Linux operating system. The -v (or --invert-match) option filters out matches. grep pattern1 *.txt | grep -v pattern2 # Matches all lines in "*.txt" files containing "pattern1", # but ***not*** "pattern2". The -c (--count) option gives a numerical count of matches, rather than actually listing the matches. grep -c txt *.sgml # (number of occurrences of "txt" in "*.sgml" files) # grep -cz . # ^ dot # means count (-c) zero-separated (-z) items matching "." # that is, non-empty ones (containing at least 1 character). # printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz . # 4 printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '$' # 5 printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^' # 5 # printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c '$' # 9 # By default, newline chars (\n) separate items to match. # Note that the -z option is GNU "grep" specific. # Thanks, S.C. When invoked with more than one target file given, grep specifies which file contains matches. bash$ grep Linux osinfo.txt misc.txt osinfo.txt:This is a file containing information about Linux. osinfo.txt:The GPL governs the distribution of the Linux operating system. misc.txt:The Linux operating system is steadily gaining in popularity. To force grep to show the filename when searching only one target file, simply give /dev/null as the second file. bash$ grep Linux osinfo.txt /dev/null osinfo.txt:This is a file containing information about Linux. osinfo.txt:The GPL governs the distribution of the Linux operating system. If there is a successful match, grep returns an exit status of 0, which makes it useful in a condition test in a script, especially in combination with the -q option to suppress output. SUCCESS=0 # if grep lookup succeeds word=Linux filename=data.file grep -q "$word" "$filename" # The "-q" option causes nothing to echo to stdout. if [ $? -eq $SUCCESS ] then echo "$word found in $filename" else echo "$word not found in $filename" fi demonstrates how to use grep to search for a word pattern in a system logfile. &grp; egrep is the same as grep -E. This uses a somewhat different, extended set of regular expressions, which can make the search somewhat more flexible. fgrep is the same as grep -F. It does a literal string search (no regular expressions), which allegedly speeds things up a bit. agrep extends the capabilities of grep to approximate matching. The search string may differ by a specified number of characters from the resulting matches. This utility is not part of the core Linux distribution. To search compressed files, use zgrep, zegrep, or zfgrep. These also work on non-compressed files, though slower than plain grep, egrep, fgrep. They are handy for searching through a mixed set of files, some compressed, some not. To search bzipped files, use bzgrep. look look command look The command look works like grep, but does a lookup on a dictionary, a sorted word list. By default, look searches for a match in /usr/dict/words, but a different dictionary file may be specified. &lookup; sed awk sed command sed awk command awk Scripting languages especially suited for parsing text files and command output. May be embedded singly or in combination in pipes and shell scripts. sed Non-interactive stream editor, permits using many ex commands in batch mode. It finds many uses in shell scripts. awk Programmable file extractor and formatter, good for manipulating and/or extracting fields (columns) in structured text files. Its syntax is similar to C. wc wc command wc wc gives a word count on a file or I/O stream: bash $ wc /usr/doc/sed-3.02/README 20 127 838 /usr/doc/sed-3.02/README [20 lines 127 words 838 characters] wc -w gives only the word count. wc -l gives only the line count. wc -c gives only the character count. wc -L gives only the length of the longest line. Using wc to count how many .txt files are in current working directory: $ ls *.txt | wc -l # Will work as long as none of the "*.txt" files have a linefeed in their name. # Alternative ways of doing this are: # find . -maxdepth 1 -name \*.txt -print0 | grep -cz . # (shopt -s nullglob; set -- *.txt; echo $#) # Thanks, S.C. Using wc to total up the size of all the files whose names begin with letters in the range d - h bash$ wc [d-h]* | grep total | awk '{print $3}' 71832 Using wc to count the instances of the word Linux in the main source file for this book. bash$ grep Linux abs-book.sgml | wc -l 50 See also and . Certain commands include some of the functionality of wc as options. ... | grep foo | wc -l # This frequently used construct can be more concisely rendered. ... | grep -c foo # Just use the "-c" (or "--count") option of grep. # Thanks, S.C. tr tr command tr character translation filter. Must use quoting and/or brackets, as appropriate. Quotes prevent the shell from reinterpreting the special characters in tr command sequences. Brackets should be quoted to prevent expansion by the shell. Either tr "A-Z" "*" <filename or tr A-Z \* <filename changes all the uppercase letters in filename to asterisks (writes to stdout). On some systems this may not work, but tr A-Z '[**]' will. The -d option deletes a range of characters. echo "abcdef" # abcdef echo "abcdef" | tr -d b-d # aef tr -d 0-9 <filename # Deletes all digits from the file "filename". The --squeeze-repeats (or -s) option deletes all but the first instance of a string of consecutive characters. This option is useful for removing excess whitespace. bash$ echo "XXXXX" | tr --squeeze-repeats 'X' X The -c complement option inverts the character set to match. With this option, tr acts only upon those characters not matching the specified set. bash$ echo "acfdeb123" | tr -c b-d + +c+d+b++++ Note that tr recognizes POSIX character classes. bash$ echo "abcd2ef1" | tr '[:alpha:]' - ----2--1 &ex49; &lowercase; &du; &rot13; &cryptoquote; The tr utility has two historic variants. The BSD version does not use brackets (tr a-z A-Z), but the SysV one does (tr '[a-z]' '[A-Z]'). The GNU version of tr resembles the BSD one, so quoting letter ranges within brackets is mandatory. fold fold command fold A filter that wraps lines of input to a specified width. This is especially useful with the -s option, which breaks lines at word spaces (see and ). fmt fmt command fmt Simple-minded file formatter, used as a filter in a pipe to wrap long lines of text output. &ex50; See also . A powerful alternative to fmt is Kamil Toman's par utility, available from http://www.cs.berkeley.edu/~amc/Par/. ptx ptx command index The ptx [targetfile] command outputs a permuted index (cross-reference list) of the targetfile. This may be further filtered and formatted in a pipe, if necessary. col col command reverse line feed This deceptively named filter removes reverse line feeds from an input stream. It also attempts to replace whitespace with equivalent tabs. The chief use of col is in filtering the output from certain text processing utilities, such as groff and tbl. column column command column Column formatter. This filter transforms list-type text output into a pretty-printed table by inserting tabs at appropriate places. &colm; colrm colrm command colrm Column removal filter. This removes columns (characters) from a file and writes the file, lacking the range of specified columns, back to stdout. colrm 2 4 <filename removes the second through fourth characters from each line of the text file filename. If the file contains tabs or nonprintable characters, this may cause unpredictable behavior. In such cases, consider using expand and unexpand in a pipe preceding colrm. nl nl command fmt Line numbering filter. nl filename lists filename to stdout, but inserts consecutive numbers at the beginning of each non-blank line. If filename omitted, operates on stdin. The output of nl is very similar to cat -n, however, by default nl does not list blank lines. &lnum; pr pr command pr Print formatting filter. This will paginate files (or stdout) into sections suitable for hard copy printing or viewing on screen. Various options permit row and column manipulation, joining lines, setting margins, numbering lines, adding page headers, and merging files, among other things. The pr command combines much of the functionality of nl, paste, fold, column, and expand. pr -o 5 --width=65 fileZZZ | more gives a nice paginated listing to screen of fileZZZ with margins set at 5 and 65. A particularly useful option is -d, forcing double-spacing (same effect as sed -G). gettext gettext command gettext A GNU utility for localization and translating the text output of programs into foreign languages. While primarily intended for C programs, gettext also finds use in shell scripts. See the info page. iconv iconv command encoding A utility for converting file(s) to a different encoding (character set). Its chief use is for localization. recode recode command encoding Consider this a fancier version of iconv, above. This very versatile utility for converting a file to a different encoding is not part of the standard Linux installation. TeX gs TeX command TeX gs command Postscript TeX and Postscript are text markup languages used for preparing copy for printing or formatted video display. TeX is Donald Knuth's elaborate typsetting system. It is often convenient to write a shell script encapsulating all the options and arguments passed to one of these markup languages. Ghostscript (gs) is a GPL-ed Postscript interpreter. groff tbl eqn groff command groff tbl command table eqn command equation Yet another text markup and display formatting language is groff. This is the enhanced GNU version of the venerable UNIX roff/troff display and typesetting package. Manpages use groff (see ). The tbl table processing utility is considered part of groff, as its function is to convert table markup into groff commands. The eqn equation processing utility is likewise part of groff, and its function is to convert equation markup into groff commands. lex yacc lex command flex yacc command bison The lex lexical analyzer produces programs for pattern matching. This has been replaced by the nonproprietary flex on Linux systems. The yacc utility creates a parser based on a set of specifications. This has been replaced by the nonproprietary bison on Linux systems. tar tar command tar The standard UNIX archiving utility. Originally a Tape ARchiving program, it has developed into a general purpose package that can handle all manner of archiving with all types of destination devices, ranging from tape drives to regular files to even stdout (see ). GNU tar has long since been patched to accept gzip compression options, such as tar czvf archive-name.tar.gz *, which recursively archives and compresses all files in a directory tree except dotfiles in the current working directory ($PWD). A tar czvf ... will include dotfiles in directories below the current working directory. This is an undocumented tar feature. Some useful tar options: -c create (a new archive) --delete delete (files from the archive) -r append (files to the archive) -t list (archive contents) -u update archive -x extract (files from the archive) -z gzip the archive It may be difficult to recover data from a corrupted gzipped tar archive. When archiving important files, make multiple backups. shar shar command archive Shell archiving utility. The files in a shell archive are concatenated without compression, and the resultant archive is essentially a shell script, complete with #!/bin/sh header, and containing all the necessary unarchiving commands. Shar archives still show up in Internet newsgroups, but otherwise shar has been pretty well replaced by tar/gzip. The unshar command unpacks shar archives. ar ar command archive Creation and manipulation utility for archives, mainly used for binary object file libraries. cpio cpio command cpio This specialized archiving copy command (copy input and output) is rarely seen any more, having been supplanted by tar/gzip. It still has its uses, such as moving a directory tree. &ex48; &derpm; gzip gzip command gzip The standard GNU/UNIX compression utility, replacing the inferior and proprietary compress. The corresponding decompression command is gunzip, which is the equivalent of gzip -d. The zcat filter decompresses a gzipped file to stdout, as possible input to a pipe or redirection. This is, in effect, a cat command that works on compressed files (including files processed with the older compress utility). The zcat command is equivalent to gzip -dc. On some commercial UNIX systems, zcat is a synonym for uncompress -c, and will not work on gzipped files. See also . bzip2 bzip2 command bzip2 An alternate compression utility, usually more efficient than gzip, especially on large files. The corresponding decompression command is bunzip2. compress uncompress compress command compress uncompress command uncompress This is an older, proprietary compression utility found in commercial UNIX distributions. The more efficient gzip has largely replaced it. Linux distributions generally include a compress workalike for compatibility, although gunzip can unarchive files treated with compress. The znew command transforms compressed files into gzipped ones. sq sq command sq Yet another compression utility, a filter that works only on sorted ASCII word lists. It uses the standard invocation syntax for a filter, sq < input-file > output-file. Fast, but not nearly as efficient as gzip. The corresponding uncompression filter is unsq, invoked like sq. The output of sq may be piped to gzip for further compression. zip unzip zip command zip unzip command unzip Cross-platform file archiving and compression utility compatible with DOS PKZIP. Zipped archives seem to be a more acceptable medium of exchange on the Internet than tarballs. file file command file A utility for identifying file types. The command file file-name will return a file specification for file-name, such as ascii text or data. It references the magic numbers found in /usr/share/magic, /etc/magic, or /usr/lib/magic, depending on the Linux/UNIX distribution. The -f option causes file to run in batch mode, to read from a designated file a list of filenames to analyze. The -z option, when used on a compressed target file, forces an attempt to analyze the uncompressed file type. bash$ file test.tar.gz test.tar.gz: gzip compressed data, deflated, last modified: Sun Sep 16 13:34:51 2001, os: Unix bash file -z test.tar.gz test.tar.gz: GNU tar archive (gzip compressed data, deflated, last modified: Sun Sep 16 13:34:51 2001, os: Unix) &stripc; which which command which which command-xxx gives the full path to command-xxx. This is useful for finding out whether a particular command or utility is installed on the system. $bash which rm /usr/bin/rm whereis whereis command whereis Similar to which, above, whereis command-xxx gives the full path to command-xxx, but also to its manpage. $bash whereis rm rm: /bin/rm /usr/share/man/man1/rm.1.bz2 whatis whatis command whatis whatis filexxx looks up filexxx in the whatis database. This is useful for identifying system commands and important configuration files. Consider it a simplified man command. $bash whatis whatis whatis (1) - search the whatis database for complete words &what; See also . vdir vdir command ls Show a detailed directory listing. The effect is similar to ls -l. This is one of the GNU fileutils. bash$ vdir total 10 -rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo -rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak -rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo bash ls -l total 10 -rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo -rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak -rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo shred shred command secure delete Securely erase a file by overwriting it multiple times with random bit patterns before deleting it. This command has the same effect as , but does it in a more thorough and elegant manner. This is one of the GNU fileutils. Using shred on a file may not prevent recovery of some or all of its contents using advanced forensic technology. locate slocate locate command locate slocate command slocate The locate command searches for files using a database stored for just that purpose. The slocate command is the secure version of locate (which may be aliased to slocate). $bash locate hickson /usr/lib/xephem/catalogs/hickson.edb strings strings command strings Use the strings command to find printable strings in a binary or data file. It will list sequences of printable characters found in the target file. This might be handy for a quick 'n dirty examination of a core dump or for looking at an unknown graphic image file (strings image-file | more might show something like JFIF, which would identify the file as a jpeg graphic). In a script, you would probably parse the output of strings with grep or sed. See and . &wstrings; basename basename command basename Strips the path information from a file name, printing only the file name. The construction basename $0 lets the script know its name, that is, the name it was invoked by. This can be used for usage messages if, for example a script is called with missing arguments: echo "Usage: `basename $0` arg1 arg2 ... argn" dirname dirname command dirname Strips the basename from a filename, printing only the path information. basename and dirname can operate on any arbitrary string. The argument does not need to refer to an existing file, or even be a filename for that matter (see ). &ex35; split split command split Utility for splitting a file into smaller chunks. Usually used for splitting up large files in order to back them up on floppies or preparatory to e-mailing or uploading them. sum cksum md5sum sum command sum cksum command cksum md5sum command md5sum These are utilities for generating checksums. A checksum is a number mathematically calculated from the contents of a file, for the purpose of checking its integrity. A script might refer to a list of checksums for security purposes, such as ensuring that the contents of key system files have not been altered or corrupted. The md5sum command is the most appropriate of these in security applications. Note that cksum also shows the size, in bytes, of the target file. bash$ cksum /boot/vmlinuz 1670054224 804083 /boot/vmlinuz bash$ md5sum /boot/vmlinuz 0f43eccea8f09e0a0b2b5cf1dcf333ba /boot/vmlinuz uuencode uuencode command uuencode This utility encodes binary files into ASCII characters, making them suitable for transmission in the body of an e-mail message or in a newsgroup posting. uudecode uudecode command uudecode This reverses the encoding, decoding uuencoded files back into the original binaries. &ex52; The fold -s command may be useful (possibly in a pipe) to process long uudecoded text messages downloaded from Usenet newsgroups. mimencode mmencode mimencode command mime mmencode command encode The mimencode and mmencode commands process multimedia-encoded e-mail attachments. Although mail user agents (such as pine or kmail) normally handle this automatically, these particular utilities permit manipulating such attachments manually from the command line or in a batch by means of a shell script. crypt crypt command crypt At one time, this was the standard UNIX file encryption utility. This is a symmetric block cipher, used to encrypt files on a single system or local network, as opposed to the public key cipher class, of which pgp is a well-known example. Politically motivated government regulations prohibiting the export of encryption software resulted in the disappearance of crypt from much of the UNIX world, and it is still missing from most Linux distributions. Fortunately, programmers have come up with a number of decent alternatives to it, among them the author's very own cruft (see ). make make command Makefile Utility for building and compiling binary packages. This can also be used for any set of operations that is triggered by incremental changes in source files. The make command checks a Makefile, a list of file dependencies and operations to be carried out. install install command install Special purpose file copying command, similar to cp, but capable of setting permissions and attributes of the copied files. This command seems tailormade for installing software packages, and as such it shows up frequently in Makefiles (in the make install : section). It could likewise find use in installation scripts. more less more command more less command less Pagers that display a text file or stream to stdout, one screenful at a time. These may be used to filter the output of a script. host host command host Searches for information about an Internet host by name or IP address, using DNS. vrfy vrfy command vrfy Verify an Internet e-mail address. nslookup nslookup command name server lookup Do an Internet name server lookup on a host by IP address. This may be run either interactively or noninteractively, i.e., from within a script. dig dig command domain information groper Similar to nslookup, do an Internet name server lookup on a host. May be run either interactively or noninteractively, i.e., from within a script. traceroute traceroute command traceroute Trace the route taken by packets sent to a remote host. This command works within a LAN, WAN, or over the Internet. The remote host may be specified by an IP address. The output of this command may be filtered by grep or sed in a pipe. ping ping command ping Broadcast an ICMP ECHO_REQUEST packet to other machines, either on a local or remote network. This is a diagnostic tool for testing network connections, and it should be used with caution. A successful ping returns an exit status of 0. This can be tested for in a script. bash$ ping localhost PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data. Warning: time of day goes back, taking countermeasures. 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec --- localhost.localdomain ping statistics --- 2 packets transmitted, 2 packets received, 0% packet loss round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms whois whois command domain name server Perform a DNS (Domain Name System) lookup. The -h option permits specifying which whois server to query. See . finger finger command finger Retrieve information about a particular user on a network. Optionally, this command can display the user's ~/.plan, ~/.project, and ~/.forward files, if present. bash$ finger bozo Login: bozo Name: Bozo Bozeman Directory: /home/bozo Shell: /bin/bash On since Fri Aug 31 20:13 (MST) on tty1 1 hour 38 minutes idle On since Fri Aug 31 20:13 (MST) on pts/0 12 seconds idle On since Fri Aug 31 20:13 (MST) on pts/1 On since Fri Aug 31 20:31 (MST) on pts/2 1 hour 16 minutes idle No mail. No Plan. Out of security considerations, many networks disable finger and its associated daemon. A daemon is a background process not attached to a terminal session. Daemons perform designated services either at specified times or explicitly triggered by certain events. The word daemon means ghost in Greek, and there is certainly something mysterious, almost supernatural, about the way UNIX daemons silently wander about behind the scenes, carrying out their appointed tasks. sx rx sx command sx rx command rx The sx and rx command set serves to transfer files to and from a remote host using the xmodem protocol. These are generally part of a communications package, such as minicom. sz rz sz command sz rz command rz The sz and rz command set serves to transfer files to and from a remote host using the zmodem protocol. Zmodem has certain advantages over xmodem, such as greater transmission rate and resumption of interrupted file transfers. Like sx and rx, these are generally part of a communications package. ftp ftp command file transfer Utility and protocol for uploading / downloading files to / from a remote host. An ftp session can be automated in a script (see , , and ). cu cu command call up Call Up a remote system and connect as a simple terminal. This is a sort of dumbed-down version of telnet. uucp uucp command uucp UNIX to UNIX copy. This is a communications package for transferring files between UNIX servers. A shell script is an effective way to handle a uucp command sequence. Since the advent of the Internet and e-mail, uucp seems to have faded into obscurity, but it still exists and remains perfectly workable in situations where an Internet connection is not available or appropriate. telnet telnet command telnet Utility and protocol for connecting to a remote host. The telnet protocol contains security holes and should therefore probably be avoided. rlogin rlogin command remote login Remote login, initates a session on a remote host. This command has security issues, so use ssh instead. rsh rsh command remote shell Remote shell, executes command(s) on a remote host. This has security issues, so use ssh instead. rcp rcp command remote copy Remote copy, copies files between two different networked machines. Using rcp and similar utilities with security implications in a shell script may not be advisable. Consider, instead, using ssh or an expect script. ssh ssh command secure shell Secure shell, logs onto a remote host and executes commands there. This secure replacement for telnet, rlogin, rcp, and rsh uses identity authentication and encryption. See its manpage for details. write write command write This is a utility for terminal-to-terminal communication. It allows sending lines from your terminal (console or xterm) to that of another user. The mesg command may, of course, be used to disable write access to a terminal Since write is interactive, it would not normally find use in a script. mail mail command mail Send an e-mail message to a user. This stripped-down command-line mail client works fine as a command embedded in a script. &selfmailer; vacation vacation command mail This utility automatically replies to e-mails that the intended recipient is on vacation and temporarily unavailable. This runs on a network, in conjunction with sendmail, and is not applicable to a dial-up POPmail account. tput tput command terminal Initialize terminal and/or fetch information about it from terminfo data. Various options permit certain terminal operations. tput clear is the equivalent of clear, below. tput reset is the equivalent of reset, below. bash$ tput longname xterm terminal emulator (XFree86 4.0 Window System) Note that stty offers a more powerful command set for controlling a terminal. reset reset command reset Reset terminal parameters and clear text screen. As with clear, the cursor and prompt reappear in the upper lefthand corner of the terminal. clear clear command clear The clear command simply clears the text screen at the console or in an xterm. The prompt and cursor reappear at the upper lefthand corner of the screen or xterm window. This command may be used either at the command line or in a script. See . script script command script This utility records (saves to a file) all the user keystrokes at the command line in a console or an xterm window. This, in effect, create a record of a session. factor factor command factor Decompose an integer into prime factors. bash$ factor 27417 27417: 3 13 19 37 bc dc bc command bc dc command dc These are flexible, arbitrary precision calculation utilities. bc has a syntax vaguely resembling C. dc uses RPN (Reverse Polish Notation). Of the two, bc seems more useful in scripting. It is a fairly well-behaved UNIX utility, and may therefore be used in a pipe. Bash can't handle floating point calculations, and it lacks operators for certain important mathematical functions. Fortunately, bc comes to the rescue. Here is a simple template for using bc to calculate a script variable. This uses command substitution. variable=$(echo "OPTIONS; OPERATIONS" | bc) &monthlypmt; &base; An alternate method of invoking bc involves using a here document embedded within a command substitution block. This is especially appropriate when a script needs to pass a list of options and commands to bc. variable=`bc << LIMIT_STRING options statements operations LIMIT_STRING ` ...or... variable=$(bc << LIMIT_STRING options statements operations LIMIT_STRING ) &altbc; awk awk command math Yet another way of doing floating point math in a script is using awk's built-in math functions in a shell wrapper. &hypot; jot seq jot command jot seq command seq loop arguments These utilities emit a sequence of integers, with a user-selected increment. The normal separator character between each integer is a newline, but this can be changed with the -s option. bash$ seq 5 1 2 3 4 5 bash$ seq -s : 5 1:2:3:4:5 Both jot and seq come in handy in a for loop. &ex53; run-parts run-parts command run-parts The run-parts command This is actually a script adapted from the Debian Linux distribution. executes all the scripts in a target directory, sequentially in ASCII-sorted filename order. Of course, the scripts need to have execute permission. The crond daemon invokes run-parts to run the scripts in the /etc/cron.* directories. yes yes command yes In its default behavior the yes command feeds a continuous string of the character y followed by a line feed to stdout. A controlc terminates the run. A different output string may be specified, as in yes different string, which would continually output different string to stdout. One might well ask the purpose of this. From the command line or in a script, the output of yes can be redirected or piped into a program expecting user input. In effect, this becomes a sort of poor man's version of expect. yes | fsck /dev/hda1 runs fsck non-interactively (careful!). yes | rm -r dirname has same effect as rm -rf dirname (careful!). Be very cautious when piping yes to a potentially dangerous system command, such as fsck or fdisk. banner banner command banner Prints arguments as a large vertical banner to stdout, using an ASCII character (default '#'). This may be redirected to a printer for hardcopy. printenv printenv command environment Show all the environmental variables set for a particular user. bash$ printenv | grep HOME HOME=/home/bozo lp lp command lpr The lp and lpr commands send file(s) to the print queue, to be printed as hard copy. The print queue is the group of jobs waiting in line to be printed. These commands trace the origin of their names to the line printers of another era. bash$ lp file1.txt or bash lp <file1.txt It is often useful to pipe the formatted output from pr to lp. bash$ pr -options file1.txt | lp Formatting packages, such as groff and Ghostscript may send their output directly to lp. bash$ groff -Tascii file.tr | lp bash$ gs -options | lp file.ps Related commands are lpq, for viewing the print queue, and lprm, for removing jobs from the print queue. tee tee command tee [UNIX borrows an idea here from the plumbing trade.] This is a redirection operator, but with a difference. Like the plumber's tee, it permits siponing off the output of a command or commands within a pipe, but without affecting the result. This is useful for printing an ongoing process to a file or paper, perhaps to keep track of it for debugging purposes. tee |------> to file | ===============|=============== command--->----|-operator-->---> result of command(s) =============================== cat listfile* | sort | tee check.file | uniq > result.file (The file check.file contains the concatenated sorted listfiles, before the duplicate lines are removed by uniq.) mkfifo mkfifo command mkfifo This obscure command creates a named pipe, a temporary first-in-first-out buffer for transferring data between processes. For an excellent overview of this topic, see Andy Vaught's article, Introduction to Named Pipes, in the September, 1997 issue of Linux Journal. Typically, one process writes to the FIFO, and the other reads from it. See . pathchk pathchk command pathchk This command checks the validity of a filename. If the filename exceeds the maximum allowable length (255 characters) or one or more of the directories in its path is not searchable, then an error message results. Unfortunately, pathchk does not return a recognizable error code, and it is therefore pretty much useless in a script. dd dd command dd This is the somewhat obscure and much feared data duplicator command. Originally a utility for exchanging data on magnetic tapes between UNIX minicomputers and IBM mainframes, this command still has its uses. The dd command simply copies a file (or stdin/stdout), but with conversions. Possible conversions are ASCII/EBCDIC, EBCDIC (pronounced ebb-sid-ic) is an acronym for Extended Binary Coded Decimal Interchange Code. This is an IBM data format no longer in much use. A bizarre application of the conv=ebcdic option of dd is as a quick 'n easy, but not very secure text file encoder. cat $file | dd conv=swab,ebcdic > $file_encrypted # Encode (looks like gibberish). # Might as well switch bytes (swab), too, for a little extra obscurity. cat $file_encrypted | dd conv=swab,ascii > $file_plaintext # Decode. upper/lower case, swapping of byte pairs between input and output, and skipping and/or truncating the head or tail of the input file. A dd --help lists the conversion and other options that this powerful utility takes. # Exercising 'dd'. n=3 p=5 input_file=project.txt output_file=log.txt dd if=$input_file of=$output_file bs=1 skip=$((n-1)) count=$((p-n+1)) 2> /dev/null # Extracts characters n to p from file $input_file. echo -n "hello world" | dd cbs=1 conv=unblock 2> /dev/null # Echoes "hello world" vertically. # Thanks, S.C. To demonstrate just how versatile dd is, let's use it to capture keystrokes. &ddkeypress; The dd command can do random access on a data stream. echo -n . | dd bs=1 seek=4 of=file conv=notrunc # The "conv=notrunc" option means that the output file will not be truncated. # Thanks, S.C. The dd command can copy raw data and disk images to and from devices, such as floppies and tape drives (). A common use is creating boot floppies. dd if=kernel-image of=/dev/fd0H1440 Similarly, dd can copy the entire contents of a floppy, even one formatted with a foreign OS, to the hard drive as an image file. dd if=/dev/fd0 of=/home/bozo/projects/floppy.img Other applications of dd include initializing temporary swap files () and ramdisks (). It can even do a low-level copy of an entire hard drive partition, although this is not necessarily recommended. People (with presumably nothing better to do with their time) are constantly thinking of interesting applications of dd. &blotout; od od command od The od, or octal dump filter converts input (or files) to octal (base-8) or other bases. This is useful for viewing or processing binary data files or otherwise unreadable system device files, such as /dev/urandom, and as a filter for binary data. See and . hexdump hexdump command hexadecimal Performs a hexadecimal, octal, decimal, or ASCII dump of a binary file. This command is the rough equivalent of od, above, but not nearly as useful. m4 m4 command macro A hidden treasure, m4 is a powerful macro processing filter, A macro is a symbolic constant that expands into a command string or a set of operations on parameters. virtually a complete language. Although originally written as a pre-processor for Fortran, m4 turned out to be useful as a stand-alone utility. In fact, m4 combines some of the functionality of eval, tr, and awk, in addition to its extensive macro expansion facilities. The April, 2002 issue of Linux Journal has a very nice article on m4 and its uses. &m4; The startup and shutdown scripts in /etc/rc.d illustrate the uses (and usefulness) of many of these comands. These are usually invoked by root and used for system maintenance or emergency filesystem repairs. Use with caution, as some of these commands may damage your system if misused. chown chgrp chown command chown chgrp command chgrp The chown command changes the ownership of a file or files. This command is a useful method that root can use to shift file ownership from one user to another. An ordinary user may not change the ownership of files, not even her own files. This is the case on a Linux machine or a UNIX system with disk quotas. root# chown bozo *.txt The chgrp command changes the group ownership of a file or files. You must be owner of the file(s) as well as a member of the destination group (or root) to use this operation. chgrp --recursive dunderheads *.data # The "dunderheads" group will now own all the "*.data" files #+ all the way down the $PWD directory tree (that's what "recursive" means). useradd userdel useradd command useradd userdel command userdel The useradd administrative command adds a user account to the system and creates a home directory for that particular user, if so specified. The corresponding userdel command removes a user account from the system The userdel command will fail if the particular user being deleted is still logged on. and deletes associated files. The adduser command is a synonym for useradd and is usually a symbolic link to it. id id command id The id command lists the real and effective user IDs and the group IDs of the current user. This is the counterpart to the $UID, $EUID, and $GROUPS internal Bash variables. bash$ id uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio) bash$ echo $UID 501 Also see . who who command whoami Show all users logged on to the system. bash$ who bozo tty1 Apr 27 17:45 bozo pts/0 Apr 27 17:46 bozo pts/1 Apr 27 17:47 bozo pts/2 Apr 27 17:49 The -m gives detailed information about only the current user. Passing any two arguments to who is the equivalent of who -m, as in who am i or who The Man. bash$ who -m localhost.localdomain!bozo pts/2 Apr 27 17:49 whoami is similar to who -m, but only lists the user name. bash$ whoami bozo w w command w Show all logged on users and the processes belonging to them. This is an extended version of who. The output of w may be piped to grep to find a specific user and/or process. bash$ w | grep startx bozo tty1 - 4:22pm 6:41 4.47s 0.45s startx logname logname command logname Show current user's login name (as found in /var/run/utmp). This is a near-equivalent to whoami, above. bash$ logname bozo bash$ whoami bozo However... bash$ su Password: ...... bash# whoami root bash# logname bozo su su command su Runs a program or script as a substitute user. su rjones starts a shell as user rjones. A naked su defaults to root. See . sudo sudo command sudo Runs a command as root (or another user). This may be used in a script, thus permitting a regular user to run the script. #!/bin/bash # Some commands. sudo cp /root/secretfile /home/bozo/secret # Some more commands. The file /etc/sudoers holds the names of users permitted to invoke sudo. users users command users Show all logged on users. This is the approximate equivalent of who -q. ac ac command accounting Show users' logged in time, as read from /var/log/wtmp. This is one of the GNU accounting utilities. bash$ ac total 68.08 last last command logged in List last logged in users, as read from /var/log/wtmp. This command can also show remote logins. groups groups command groups Lists the current user and the groups she belongs to. This corresponds to the $GROUPS internal variable, but gives the group names, rather than the numbers. bash$ groups bozita cdrom cdwriter audio xgrp bash$ echo $GROUPS 501 newgrp newgrp command group Change user's group ID without logging out. This permits access to the new group's files. Since users may be members of multiple groups simultaneously, this command finds little use. tty tty command tty Echoes the name of the current user's terminal. Note that each separate xterm window counts as a different terminal. bash$ tty /dev/pts/1 stty stty command stty Shows and/or changes terminal settings. This complex command, used in a script, can control terminal behavior and the way output displays. See the info page, and study it carefully. &erase; &secretpw; A creative use of stty is detecting a user keypress (without hitting ENTER). &keypress; Also see . Normally, a terminal works in the canonical mode. When a user hits a key, the resulting character does not immediately go to the program actually running in this terminal. A buffer local to the terminal stores keystrokes. When the user hits the ENTER key, this sends all the stored keystrokes to the program running. There is even a basic line editor inside the terminal. bash$ stty -a speed 9600 baud; rows 36; columns 96; line = 0; intr = ^C; quit = ^\; erase = ^H; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; ... isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt Using canonical mode, it is possible to redefine the special keys for the local terminal line editor. bash$ cat > filexxx wha<ctl-W>I<ctl-H>foo bar<ctl-U>hello world<ENTER> <ctl-D> bash$ cat filexxx hello world bash$ bash$ wc -c < file 13 The process controlling the terminal receives only 13 characters (12 alphabetic ones, plus a newline), although the user hit 26 keys. In non-canonical (raw) mode, every key hit (including special editing keys such as ctl-H) sends a character immediately to the controlling process. The Bash prompt disables both icanon and echo, since it replaces the basic terminal line editor with its own more elaborate one. For example, when you hit ctl-A at the Bash prompt, there's no ^A echoed by the terminal, but Bash gets a \1 character, interprets it, and moves the cursor to the begining of the line. Stephane Chazelas tset tset command tset Show or initialize terminal settings. This is a less capable version of stty. bash$ tset -r Terminal type is xterm-xfree86. Kill is control-U (^U). Interrupt is control-C (^C). setserial setserial command serial Set or display serial port parameters. This command must be run by root user and is usually found in a system setup script. # From /etc/pcmcia/serial script: IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'` setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ getty agetty getty command getty agetty command agetty The initialization process for a terminal uses getty or agetty to set it up for login by a user. These commands are not used within user shell scripts. Their scripting counterpart is stty. mesg mesg command mesg Enables or disables write access to the current user's terminal. Disabling access would prevent another user on the network to write to the terminal. It can be very annoying to have a message about ordering pizza suddenly appear in the middle of the text file you are editing. On a multi-user network, you might therefore wish to disable write access to your terminal when you need to avoid interruptions. wall wall command wall This is an acronym for write all, i.e., sending a message to all users at every terminal logged into the network. It is primarily a system administrator's tool, useful, for example, when warning everyone that the system will shortly go down due to a problem (see ). bash$ wall System going down for maintenance in 5 minutes! Broadcast message from bozo (pts/1) Sun Jul 8 13:53:27 2001... System going down for maintenance in 5 minutes! If write access to a particular terminal has been disabled with mesg, then wall cannot send a message to it. dmesg dmesg command dmesg Lists all system bootup messages to stdout. Handy for debugging and ascertaining which device drivers were installed and which system interrupts in use. The output of dmesg may, of course, be parsed with grep, sed, or awk from within a script. uname uname command uname Output system specifications (OS, kernel version, etc.) to stdout. Invoked with the -a option, gives verbose system info (see ). The -s option shows only the OS type. bash$ uname -a Linux localhost.localdomain 2.2.15-2.5.0 #1 Sat Feb 5 00:13:43 EST 2000 i686 unknown bash$ uname -s Linux arch arch command arch Show system architecture. Equivalent to uname -m. See . bash$ arch i686 bash$ uname -m i686 lastcomm lastcomm command last Gives information about previous commands, as stored in the /var/account/pacct file. Command name and user name can be specified by options. This is one of the GNU accounting utilities. lastlog lastlog command last List the last login time of all system users. This references the /var/log/lastlog file. bash$ lastlog root tty1 Fri Dec 7 18:43:21 -0700 2001 bin **Never logged in** daemon **Never logged in** ... bozo tty1 Sat Dec 8 21:14:29 -0700 2001 bash$ lastlog | grep root root tty1 Fri Dec 7 18:43:21 -0700 2001 This command will fail if the user invoking it does not have read permission for the /var/log/lastlog file. lsof lsof command lsof List open files. This command outputs a detailed table of all currently open files and gives information about their owner, size, the processes associated with them, and more. Of course, lsof may be piped to grep and/or awk to parse and analyze its results. bash$ lsof COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME init 1 root mem REG 3,5 30748 30303 /sbin/init init 1 root mem REG 3,5 73120 8069 /lib/ld-2.1.3.so init 1 root mem REG 3,5 931668 8075 /lib/libc-2.1.3.so cardmgr 213 root mem REG 3,5 36956 30357 /sbin/cardmgr ... strace strace command strace Diagnostic and debugging tool for tracing system calls and signals. The simplest way of invoking it is strace COMMAND. bash$ strace df execve("/bin/df", ["df"], [/* 45 vars */]) = 0 uname({sys="Linux", node="bozo.localdomain", ...}) = 0 brk(0) = 0x804f5e4 ... This is the Linux equivalent of truss. free free command free Shows memory and cache usage in tabular form. The output of this command lends itself to parsing, using grep, awk or Perl. The procinfo command shows all the information that free does, and much more. bash$ free total used free shared buffers cached Mem: 30504 28624 1880 15820 1608 16376 -/+ buffers/cache: 10640 19864 Swap: 68540 3128 65412 To show unused RAM memory: bash$ free | grep Mem | awk '{ print $4 }' 1880 procinfo procinfo command procinfo Extract and list information and statistics from the /proc pseudo-filesystem. This gives a very extensive and detailed listing. bash$ procinfo | grep Bootup Bootup: Wed Mar 21 15:15:50 2001 Load average: 0.04 0.21 0.34 3/47 6829 lsdev lsdev command device List devices, that is, show installed hardware. bash$ lsdev Device DMA IRQ I/O Ports ------------------------------------------------ cascade 4 2 dma 0080-008f dma1 0000-001f dma2 00c0-00df fpu 00f0-00ff ide0 14 01f0-01f7 03f6-03f6 ... du du command du Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified. bash$ du -ach 1.0k ./wi.sh 1.0k ./tst.sh 1.0k ./random.file 6.0k . 6.0k total df df command df Shows filesystem usage in tabular form. bash$ df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda5 273262 92607 166547 36% / /dev/hda8 222525 123951 87085 59% /home /dev/hda7 1408796 1075744 261488 80% /usr stat stat command stat Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files. bash$ stat test.cru File: "test.cru" Size: 49970 Allocated Blocks: 100 Filetype: Regular File Mode: (0664/-rw-rw-r--) Uid: ( 501/ bozo) Gid: ( 501/ bozo) Device: 3,8 Inode: 18185 Links: 1 Access: Sat Jun 2 16:40:24 2001 Modify: Sat Jun 2 16:40:24 2001 Change: Sat Jun 2 16:40:24 2001 If the target file does not exist, stat returns an error message. bash$ stat nonexistent-file nonexistent-file: No such file or directory vmstat vmstat command virtual memory Display virtual memory statistics. bash$ vmstat procs memory swap io system cpu r b w swpd free buff cache si so bi bo in cs us sy id 0 0 0 0 11040 2636 38952 0 0 33 7 271 88 8 3 89 netstat netstat command netstat Show current network statistics and information, such as routing tables and active connections. This utility accesses information in /proc/net (). See . netstat -r is equivalent to route. uptime uptime command uptime Shows how long the system has been running, along with associated statistics. bash$ uptime 10:28pm up 1:57, 3 users, load average: 0.17, 0.34, 0.27 hostname hostname command hostname Lists the system's host name. This command sets the host name in an /etc/rc.d setup script (/etc/rc.d/rc.sysinit or similar). It is equivalent to uname -n, and a counterpart to the $HOSTNAME internal variable. bash$ hostname localhost.localdomain bash$ echo $HOSTNAME localhost.localdomain hostid hostid command host id Echo a 32-bit hexadecimal numerical identifier for the host machine. bash$ hostid 7f0100 This command allegedly fetches a unique serial number for a particular system. Certain product registration procedures use this number to brand a particular user license. Unfortunately, hostid only returns the machine network address in hexadecimal, with pairs of bytes transposed. The network address of a typical non-networked Linux machine, is found in /etc/hosts. bash$ cat /etc/hosts 127.0.0.1 localhost.localdomain localhost As it happens, transposing the bytes of 127.0.0.1, we get 0.127.1.0, which translates in hex to 007f0100, the exact equivalent of what hostid returns, above. There exist only a few million other Linux machines with this identical hostid. sar sar command system activity report Invoking sar (system activity report) gives a very detailed rundown on system statistics. This command is found on some commercial UNIX systems, but is not part of the base Linux distribution. It is contained in the sysstat utilities package, written by Sebastien Godard. bash$ sar Linux 2.4.7-10 (localhost.localdomain) 12/31/2001 10:30:01 AM CPU %user %nice %system %idle 10:40:00 AM all 1.39 0.00 0.77 97.84 10:50:00 AM all 76.83 0.00 1.45 21.72 11:00:00 AM all 1.32 0.00 0.69 97.99 11:10:00 AM all 1.17 0.00 0.30 98.53 11:20:00 AM all 0.51 0.00 0.30 99.19 06:30:00 PM all 100.00 0.00 100.01 0.00 Average: all 1.39 0.00 0.66 97.95 logger logger command logger Appends a user-generated message to the system log (/var/log/messages). You do not have to be root to invoke logger. logger Experiencing instability in network connection at 23:10, 05/21. # Now, do a 'tail /var/log/messages'. By embedding a logger command in a script, it is possible to write debugging information to /var/log/messages. logger -t $0 -i Logging at line "$LINENO". # The "-t" option specifies the tag for the logger entry. # The "-i" option records the process ID. # tail /var/log/message # ... # Jul 7 20:48:58 localhost ./test.sh[1712]: Logging at line 3. logrotate logrotate command logrotate This utility manages the system log files, rotating, compressing, deleting, and/or mailing them, as appropriate. Usually crond runs logrotate on a daily basis. Adding an appropriate entry to /etc/logrotate.conf makes it possible to manage personal log files, as well as system-wide ones. ps ps command ps Process Statistics: lists currently executing processes by owner and PID (process id). This is usually invoked with ax options, and may be piped to grep or sed to search for a specific process (see and ). bash$ ps ax | grep sendmail 295 ? S 0:00 sendmail: accepting connections on port 25 pstree pstree command pstree Lists currently executing processes in tree format. The -p option shows the PIDs, as well as the process names. top top command processes Continuously updated display of most cpu-intensive processes. The -b option displays in text mode, so that the output may be parsed or accessed from a script. bash$ top -b 8:30pm up 3 min, 3 users, load average: 0.49, 0.32, 0.13 45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped CPU states: 13.6% user, 7.3% system, 0.0% nice, 78.9% idle Mem: 78396K av, 65468K used, 12928K free, 0K shrd, 2352K buff Swap: 157208K av, 0K used, 157208K free 37244K cached PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND 848 bozo 17 0 996 996 800 R 5.6 1.2 0:00 top 1 root 8 0 512 512 444 S 0.0 0.6 0:04 init 2 root 9 0 0 0 0 SW 0.0 0.0 0:00 keventd ... nice nice command nice Run a background job with an altered priority. Priorities run from 19 (lowest) to -20 (highest). Only root may set the negative (higher) priorities. Related commands are renice, snice, and skill. nohup nohup command nohup Keeps a command running even after user logs off. The command will run as a foreground process unless followed by &. If you use nohup within a script, consider coupling it with a wait to avoid creating an orphan or zombie process. pidof pidof command process id Identifies process id (pid) of a running job. Since job control commands, such as kill and renice act on the pid of a process (not its name), it is sometimes necessary to identify that pid. The pidof command is the approximate counterpart to the $PPID internal variable. bash$ pidof xclock 880 &killprocess; fuser fuser command fuser Identifies the processes (by pid) that are accessing a given file, set of files, or directory. May also be invoked with the -k option, which kills those processes. This has interesting implications for system security, especially in scripts preventing unauthorized users from accessing system services. crond crond command cron Administrative program scheduler, performing such duties as cleaning up and deleting system log files and updating the slocate database. This is the superuser version of at (although each user may have their own crontab file which can be changed with the crontab command). It runs as a daemon and executes scheduled entries from /etc/crontab. init init command init The init command is the parent of all processes. Called in the final step of a bootup, init determines the runlevel of the system from /etc/inittab. Invoked by its alias telinit, and by root only. telinit telinit command telinit Symlinked to init, this is a means of changing the system runlevel, usually done for system maintenance or emergency filesystem repairs. Invoked only by root. This command can be dangerous - be certain you understand it well before using! runlevel runlevel command runlevel Shows the current and last runlevel, that is, whether the system is halted (runlevel 0), in single-user mode (1), in multi-user mode (2 or 3), in X Windows (5), or rebooting (6). This command accesses the /var/run/utmp file. halt shutdown reboot halt command halt shutdown command shutdown reboot command reboot Command set to shut the system down, usually just prior to a power down. ifconfig ifconfig command ifconfig Network interface configuration and tuning utility. It is most often used at bootup to set up the interfaces, or to shut them down when rebooting. # Code snippets from /etc/rc.d/init.d/network # ... # Check that networking is up. [ ${NETWORKING} = "no" ] && exit 0 [ -x /sbin/ifconfig ] || exit 0 # ... for i in $interfaces ; do if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then action "Shutting down interface $i: " ./ifdown $i boot fi # The GNU-specific "-q" option to to "grep" means "quiet", i.e., producing no output. # Redirecting output to /dev/null is therefore not strictly necessary. # ... echo "Currently active devices:" echo `/sbin/ifconfig | grep ^[a-z] | awk '{print $1}'` # ^^^^^ should be quoted to prevent globbing. # The following also work. # echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })' # echo $(/sbin/ifconfig | sed -e 's/ .*//') # Thanks, S.C., for additional comments. See also . route route command route Show info about or make changes to the kernel routing table. bash$ route Destination Gateway Genmask Flags MSS Window irtt Iface pm3-67.bozosisp * 255.255.255.255 UH 40 0 0 ppp0 127.0.0.0 * 255.0.0.0 U 40 0 0 lo default pm3-67.bozosisp 0.0.0.0 UG 40 0 0 ppp0 chkconfig chkconfig command network configuration Check network configuration. This command lists and manages the network services started at bootup in the /etc/rc?.d directory. Originally a port from IRIX to Red Hat Linux, chkconfig may not be part of the core installation of some Linux flavors. bash$ chkconfig --list atd 0:off 1:off 2:off 3:on 4:on 5:on 6:off rwhod 0:off 1:off 2:off 3:off 4:off 5:off 6:off ... tcpdump tcpdump command tcp Network packet sniffer. This is a tool for analyzing and troubleshooting traffic on a network by dumping packet headers that match specified criteria. Dump ip packet traffic between hosts bozoville and caduceus: bash$ tcpdump ip host bozoville and caduceus Of course, the output of tcpdump can be parsed, using certain of the previously discussed text processing utilities. mount mount command mount Mount a filesystem, usually on an external device, such as a floppy or CDROM. The file /etc/fstab provides a handy listing of available filesystems, partitions, and devices, including options, that may be automatically or manually mounted. The file /etc/mtab shows the currently mounted filesystems and partitions (including the virtual ones, such as /proc). mount -a mounts all filesystems and partitions listed in /etc/fstab, except those with a noauto option. At bootup, a startup script in /etc/rc.d (rc.sysinit or something similar) invokes this to get everything mounted. mount -t iso9660 /dev/cdrom /mnt/cdrom # Mounts CDROM mount /mnt/cdrom # Shortcut, if /mnt/cdrom listed in /etc/fstab This versatile command can even mount an ordinary file on a block device, and the file will act as if it were a filesystem. Mount accomplishes that by associating the file with a loopback device. One application of this is to mount and examine an ISO9660 image before burning it onto a CDR. For more detail on burning CDRs, see Alex Withers' article, Creating CDs, in the October, 1999 issue of Linux Journal. # As root... mkdir /mnt/cdtest # Prepare a mount point, if not already there. mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest # Mount the image. # "-o loop" option equivalent to "losetup /dev/loop0" cd /mnt/cdtest # Now, check the image. ls -alR # List the files in the directory tree there. # And so forth. umount umount command umount Unmount a currently mounted filesystem. Before physically removing a previously mounted floppy or CDROM disk, the device must be umounted, else filesystem corruption may result. umount /mnt/cdrom # You may now press the eject button and safely remove the disk. The automount utility, if properly installed, can mount and unmount floppies or CDROM disks as they are accessed or removed. On laptops with swappable floppy and CDROM drives, this can cause problems, though. sync sync command sync Forces an immediate write of all updated data from buffers to hard drive (synchronize drive with buffers). While not strictly necessary, a sync assures the sys admin or user that the data just changed will survive a sudden power failure. In the olden days, a sync; sync (twice, just to make absolutely sure) was a useful precautionary measure before a system reboot. At times, you may wish to force an immediate buffer flush, as when securely deleting a file (see ) or when the lights begin to flicker. losetup losetup command losetup Sets up and configures loopback devices. SIZE=1000000 # 1 meg head -c $SIZE < /dev/zero > file # Set up file of designated size. losetup /dev/loop0 file # Set it up as loopback device. mke2fs /dev/loop0 # Create filesystem. mount -o loop /dev/loop0 /mnt # Mount it. # Thanks, S.C. mkswap mkswap command mkswap Creates a swap partition or file. The swap area must subsequently be enabled with swapon. swapon swapoff swapon command swapon swapoff command swapoff Enable / disable swap partitition or file. These commands usually take effect at bootup and shutdown. mke2fs mke2fs command mke2fs Create a Linux ext2 filesystem. This command must be invoked as root. &adddrv; See also and . tune2fs tune2fs command tune2fs Tune ext2 filesystem. May be used to change filesystem parameters, such as maximum mount count. This must be invoked as root. This is an extremely dangerous command. Use it at your own risk, as you may inadvertently destroy your filesystem. dumpe2fs dumpe2fs command dumpe2fs Dump (list to stdout) very verbose filesystem info. This must be invoked as root. root# dumpe2fs /dev/hda7 | grep 'ount count' dumpe2fs 1.19, 13-Jul-2000 for EXT2 FS 0.5b, 95/08/09 Mount count: 6 Maximum mount count: 20 hdparm hdparm command hard disk parameters List or change hard disk parameters. This command must be invoked as root, and it may be dangerous if misused. fdisk fdisk command fdisk Create or change a partition table on a storage device, usually a hard drive. This command must be invoked as root. Use this command with extreme caution. If something goes wrong, you may destroy an existing filesystem. fsck e2fsck debugfs fsck command fsck e2fsck command e2fsck debugfs command debugfs Filesystem check, repair, and debug command set. fsck: a front end for checking a UNIX filesystem (may invoke other utilities). The actual filesystem type generally defaults to ext2. e2fsck: ext2 filesystem checker. debugfs: ext2 filesystem debugger. One of the uses of this versatile, but dangerous command is to (attempt to) recover deleted files. For advanced users only! All of these should be invoked as root, and they can damage or destroy a filesystem if misused. badblocks badblocks command badblocks Checks for bad blocks (physical media flaws) on a storage device. This command finds use when formatting a newly installed hard drive or testing the integrity of backup media. The -c option to mke2fs also invokes a check for bad blocks. As an example, badblocks /dev/fd0 tests a floppy disk. The badblocks command may be invoked destructively (overwrite all data) or in non-destructive read-only mode. If root user owns the device to be tested, as is generally the case, then root must invoke this command. mkbootdisk mkbootdisk command bootdisk Creates a boot floppy which can be used to bring up the system if, for example, the MBR (master boot record) becomes corrupted. The mkbootdisk command is actually a Bash script, written by Erik Troan, in the /sbin directory. chroot chroot command chroot directory root change CHange ROOT directory. Normally commands are fetched from $PATH, relative to /, the default root directory. This changes the root directory to a different one (and also changes the working directory to there). This is useful for security purposes, for instance when the system administrator wishes to restrict certain users, such as those telnetting in, to a secured portion of the filesystem (this is sometimes referred to as confining a guest user to a chroot jail). Note that after a chroot, the execution path for system binaries is no longer valid. A chroot /opt would cause references to /usr/bin to be translated to /opt/usr/bin. Likewise, chroot /aaa/bbb /bin/ls would redirect future instances of ls to /aaa/bbb as the base directory, rather than / as is normally the case. An alias XX 'chroot /aaa/bbb ls' in a user's ~/.bashrc effectively restricts which portion of the filesystem she may run command XX on. The chroot command is also handy when running from an emergency boot floppy (chroot to /dev/fd0), or as an option to lilo when recovering from a system crash. Other uses include installation from a different filesystem (an rpm option) or running a readonly filesystem from a CD ROM. Invoke only as root, and use with care. It might be necessary to copy certain system files to a chrooted directory, since the normal $PATH can no longer be relied upon. lockfile lockfile command lockfile This utility is part of the procmail package (www.procmail.org). It creates a lock file, a semaphore file that controls access to a file, device, or resource. The lock file serves as a flag that this particular file, device, or resource is in use by a particular process (busy), and this permits only restricted access (or no access) to other processes. Lock files are used in such applications as protecting system mail folders from simultaneously being changed by multiple users, indicating that a modem port is being accessed, and showing that an instance of Netscape is using its cache. Scripts may check for the existence of a lock file created by a certain process to check if that process is running. Note that if a script attempts create a lock file that already exists, the script will likely hang. Normally, applications create and check for lock files in the /var/lock directory. A script can test for the presence of a lock file by something like the following. appname=xyzip # Application "xyzip" created lock file "/var/lock/xyzip.lock". if [ -e "/var/lock/$appname.lock ] then ... mknod mknod command mknod Creates block or character device files (may be necessary when installing new hardware on the system). tmpwatch tmpwatch command tmpwatch Automatically deletes files which have not been accessed within a specified period of time. Usually invoked by crond to remove stale log files. MAKEDEV MAKEDEV command make device file Utility for creating device files. It must be run as root, and in the /dev directory. root# ./MAKEDEV This is a sort of advanced version of mknod. dump restore dump command dump restore command restore The dump command is an elaborate filesystem backup utility, generally used on larger installations and networks. Operators of single-user Linux systems generally prefer something simpler for backups, such as tar. It reads raw disk partitions and writes a backup file in a binary format. Files to be backed up may be saved to a variety of storage media, including disks and tape drives. The restore command restores backups made with dump. fdformat fdformat command floppy Perform a low-level format on a floppy disk. ulimit ulimit command ulimit Sets an upper limit on system resources. Usually invoked with the -f option, which sets a limit on file size (ulimit -f 1000 limits files to 1 meg maximum). The -t option limits the coredump size (ulimit -c 0 eliminates coredumps). Normally, the value of ulimit would be set in /etc/profile and/or ~/.bash_profile (see ). umask command umask umask User file creation MASK. Limit the default file attributes for a particular user. All files created by that user take on the attributes specified by umask. The (octal) value passed to umask defines the the file permissions disabled. For example, umask 022 ensures that new files will have at most 755 permissions (777 NAND 022). NAND is the logical not-and operator. Its effect is somewhat similar to subtraction. Of course, the user may later change the attributes of particular files with chmod.The usual practice is to set the value of umask in /etc/profile and/or ~/.bash_profile (see ). rdev rdev command rdev Get info about or make changes to root device, swap space, or video mode. The functionality of rdev has generally been taken over by lilo, but rdev remains useful for setting up a ram disk. This is another dangerous command, if misused. lsmod lsmod command lsmod List installed kernel modules. bash$ lsmod Module Size Used by autofs 9456 2 (autoclean) opl3 11376 0 serial_cs 5456 0 (unused) sb 34752 0 uart401 6384 0 [sb] sound 58368 0 [opl3 sb uart401] soundlow 464 0 [sound] soundcore 2800 6 [sb sound] ds 6448 2 [serial_cs] i82365 22928 2 pcmcia_core 45984 0 [serial_cs ds i82365] insmod insmod command insmod Force installation of a kernel module. Must be invoked as root. rmmod rmmod command rmmod Force unloading of a kernel module. Must be invoked as root. modprobe modprobe command modprobe Module loader that is normally invoked automatically in a startup script. depmod depmod command depmod Creates module dependency file, usually invoked from startup script. env env command env Runs a program or script with certain environmental variables set or changed (without changing the overall system environment). The [varname=xxx] permits changing the environmental variable varname for the duration of the script. With no options specified, this command lists all the environmental variable settings. In Bash and other Bourne shell derivatives, it is possible to set variables in a single command's environment. var1=value1 var2=value2 commandXXX # $var1 and $var2 set in the environment of 'commandXXX' only. The first line of a script (the sha-bang line) may use env when the path to the shell or interpreter is unknown. #! /usr/bin/env perl print "This Perl script will run,\n"; print "even when I don't know where to find Perl.\n"; # Good for portable cross-platform scripts, # where the Perl binaries may not be in the expected place. # Thanks, S.C. ldd ldd command ldd Show shared lib dependencies for an executable file. bash$ ldd /bin/ls libc.so.6 => /lib/libc.so.6 (0x4000c000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000) strip strip command symbol Remove the debugging symbolic references from an executable binary. This decreases its size, but makes debugging of it impossible. This command often occurs in a Makefile, but rarely in a shell script. nm nm command symbol List symbols in an unstripped compiled binary. rdist rdist command rdist Remote distribution client: synchronizes, clones, or backs up a file system on a remote server. Using our knowledge of administrative commands, let us examine a system script. One of the shortest and simplest to understand scripts is killall, used to suspend running processes at system shutdown. &ex55; That wasn't so bad. Aside from a little fancy footwork with variable matching, there is no new material there. In /etc/rc.d/init.d, analyze the halt script. It is a bit longer than killall, but similar in concept. Make a copy of this script somewhere in your home directory and experiment with it (do not run it as root). Do a simulated run with the -vn flags (sh -vn scriptname). Add extensive comments. Change the action commands to echos. Look at some of the more complex scripts in /etc/rc.d/init.d. See if you can understand parts of them. Follow the above procedure to analyze them. For some additional insight, you might also examine the file sysvinitfiles in /usr/share/doc/initscripts-?.??, which is part of the initscripts documentation. $ special character ` Command substitution reassigns the output of a command For purposes of command substitution, a command may be an external system command, an internal scripting builtin, or even a script function. or even multiple commands; it literally plugs the command output into another context. The classic form of command substitution uses backquotes (`...`). Commands within backquotes (backticks) generate command line text. script_name=`basename $0` echo "The name of this script is $script_name." rm `cat filename` # filename contains a list of files to delete. # # S. C. points out that "arg list too long" error might result. # Better is xargs rm -- < filename # ( -- covers those cases where filename begins with a - ) textfile_listing=`ls *.txt` # Variable contains names of all *.txt files in current working directory. echo $textfile_listing textfile_listing2=$(ls *.txt) # The alternative form of command substitution. echo $textfile_listing # Same result. # A possible problem with putting a list of files into a single string # is that a newline may creep in. # # A safer way to assign a list of files to a parameter is with an array. # shopt -s nullglob # If no match, filename expands to nothing. # textfile_listing=( *.txt ) # # Thanks, S.C. Command substitution may result in word splitting. COMMAND `echo a b` # 2 args: a and b COMMAND "`echo a b`" # 1 arg: "a b" COMMAND `echo` # no arg COMMAND "`echo`" # one empty arg # Thanks, S.C. Even when there is no word splitting, command substitution can remove trailing newlines. # cd "`pwd`" # This should always work. # However... mkdir 'dir with trailing newline ' cd 'dir with trailing newline ' cd "`pwd`" # Error message: # bash: cd: /tmp/file with trailing newline: No such file or directory cd "$PWD" # Works fine. old_tty_setting=$(stty -g) # Save old terminal setting. echo "Hit a key " stty -icanon -echo # Disable "canonical" mode for terminal. # Also, disable *local* echo. key=$(dd bs=1 count=1 2> /dev/null) # Using 'dd' to get a keypress. stty "$old_tty_setting" # Restore old setting. echo "You hit ${#key} key." # ${#variable} = number of characters in $variable # # Hit any key except RETURN, and the output is "You hit 1 key." # Hit RETURN, and it's "You hit 0 key." # The newline gets eaten in the command substitution. Thanks, S.C. Using echo to output an unquoted variable set with command substitution removes trailing newlines characters from the output of the reassigned command(s). This can cause unpleasant surprises. dir_listing=`ls -l` echo $dir_listing # unquoted # Expecting a nicely ordered directory listing. # However, what you get is: # total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo # bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh # The newlines disappeared. echo "$dir_listing" # quoted # -rw-rw-r-- 1 bozo 30 May 13 17:15 1.txt # -rw-rw-r-- 1 bozo 51 May 15 20:57 t2.sh # -rwxr-xr-x 1 bozo 217 Mar 5 21:13 wi.sh Command substitution even permits setting a variable to the contents of a file, using either redirection or the cat command. variable1=`<file1` # Set "variable1" to contents of "file1". variable2=`cat file2` # Set "variable2" to contents of "file2". # Be aware that the variables may contain embedded whitespace, #+ or even (horrors), control characters. # Excerpts from system file, /etc/rc.d/rc.sysinit #+ (on a Red Hat Linux installation) if [ -f /fsckoptions ]; then fsckoptions=`cat /fsckoptions` ... fi # # if [ -e "/proc/ide/${disk[$device]}/media" ] ; then hdmedia=`cat /proc/ide/${disk[$device]}/media` ... fi # # if [ ! -n "`uname -r | grep -- "-"`" ]; then ktag="`cat /proc/version`" ... fi # # if [ $usb = "1" ]; then sleep 5 mouseoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=02"` kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=01"` ... fi Do not set a variable to the contents of a long text file unless you have a very good reason for doing so. Do not set a variable to the contents of a binary file, even as a joke. &stupscr; Notice that a buffer overrun does not occur. This is one instance where an interpreted language, such as Bash, provides more protection from programmer mistakes than a compiled language. Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output of an echo command within the loop. &csubloop; Command substitution makes it possible to extend the toolset available to Bash. It is simply a matter of writing a program or script that outputs to stdout (like a well-behaved UNIX tool should) and assigning that output to a variable. #include <stdio.h> /* "Hello, world." C program */ int main() { printf( "Hello, world." ); return (0); } bash$ gcc -o hello hello.c #!/bin/bash # hello.sh greeting=`./hello` echo $greeting bash$ sh hello.sh Hello, world. The $(COMMAND) form has superseded backticks for command substitution. output=$(sed -n /"$1"/p $file) # From "grp.sh" example. Examples of command substitution in shell scripts: Arithmetic expansion provides a powerful tool for performing arithmetic operations in scripts. Translating a string into a numerical expression is relatively straightforward using backticks, double parentheses, or let. Arithmetic expansion with backticks (often used in conjunction with expr) arithmetic expansion arithmetic expansion z=`expr $z + 3` # 'expr' does the expansion. Arithmetic expansion with double parentheses double parentheses and using let let let The use of backticks in arithmetic expansion has been superseded by double parentheses $((...)) or the very convenient let construction. z=$(($z+3)) # $((EXPRESSION)) is arithmetic expansion. # Not to be confused with # command substitution. let z=z+3 let "z += 3" #If quotes, then spaces and special operators allowed. # 'let' is actually arithmetic evaluation, rather than expansion. All the above are equivalent. You may use whichever one rings your chimes. Examples of arithmetic expansion in scripts: There are always three default files open, stdin (the keyboard), stdout (the screen), and stderr (error messages output to the screen). These, and any other open files, can be redirected. Redirection simply means capturing output from a file, command, program, script, or even code block within a script (see and ) and sending it as input to another file, command, program, or script. Each open file gets assigned a file descriptor. A file descriptor is simply a number that the operating system assigns to an open file to keep track of it. Consider it a simplified version of a file pointer. It is analogous to a file handle in C. The file descriptors for stdin, stdout, and stderr are 0, 1, and 2, respectively. For opening additional files, there remain descriptors 3 to 9. It is sometimes useful to assign one of these additional file descriptors to stdin, stdout, or stderr as a temporary duplicate link. Using file descriptor 5 might cause problems. When Bash creates a child process, as with exec, the child inherits fd 5 (see Chet Ramey's archived e-mail, SUBJECT: RE: File descriptor 5 is held open). Best leave this particular fd alone. This simplifies restoration to normal after complex redirection and reshuffling (see ). > # Redirect stdout to a file. # Creates the file if not present, otherwise overwrites it. ls -lR > dir-tree.list # Creates a file containing a listing of the directory tree. : > filename # The > truncates file "filename" to zero length. # If file not present, creates zero-length file (same effect as 'touch'). # The : serves as a dummy placeholder, producing no output. >> # Redirect stdout to a file. # Creates the file if not present, otherwise appends to it. # Single-line redirection commands (affect only the line they are on): # -------------------------------------------------------------------- 1>filename # Redirect stdout to file "filename". 1>>filename # Redirect and append stdout to file "filename". 2>filename # Redirect stderr to file "filename". 2>>filename # Redirect and append stderr to file "filename". #============================================================================== # Redirecting stdout, one line at a time. LOGFILE=script.log echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE echo "This statement is echoed to stdout, and will not appear in \"$LOGFILE\"." # These redirection commands automatically "reset" after each line. # Redirecting stderr, one line at a time. ERRORFILE=script.errors bad_command1 2>$ERRORFILE # Error message sent to $ERRORFILE. bad_command2 2>>$ERRORFILE # Error message appended to $ERRORFILE. bad_command3 # Error message echoed to stderr, #+ and does not appear in $ERRORFILE. # These redirection commands also automatically "reset" after each line. #============================================================================== 2>&1 # Redirects stderr to stdout. # Error messages get sent to same place as standard output. i>&j # Redirects file descriptor i to j. # All output of file pointed to by i gets sent to file pointed to by j. >&j # Redirects, by default, file descriptor 1 (stdout) to j. # All stdout gets sent to file pointed to by j. 0< < # Accept input from a file. # Companion command to >, and often used in combination with it. # # grep search-word <filename [j]<>filename # Open file "filename" for reading and writing, and assign file descriptor "j" to it. # If "filename" does not exist, create it. # If file descriptor "j" is not specified, default to fd 0, stdin. # # An application of this is writing at a specified place in a file. echo 1234567890 > File # Write string to "File". exec 3<> File # Open "File" and assign fd 3 to it. read -n 4 <&3 # Read only 4 characters. echo -n . >&3 # Write a decimal point there. exec 3>&- # Close fd 3. cat File # ==> 1234.67890 # Random access, by golly. | # Pipe. # General purpose process and command chaining tool. # Similar to >, but more general in effect. # Useful for chaining commands, scripts, files, and programs together. cat *.txt | sort | uniq > result-file # Sorts the output of all the .txt files and deletes duplicate lines, # finally saves results to result-file. Multiple instances of input and output redirection and/or pipes can be combined in a single command line. command < input-file > output-file command1 | command2 | command3 > output-file See and . Multiple output streams may be redirected to one file. ls -yz >> command.log 2>&1 # Capture result of illegal options "yz" to "ls" in file "command.log". # Because stderr redirected to the file, any error messages will also be there. n<&- Close input file descriptor n. 0<&- <&- Close stdin. n>&- Close output file descriptor n. 1>&- >&- Close stdout. Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited, close it. # Redirecting only stderr to a pipe. exec 3>&1 # Save current "value" of stdout. ls -l 2>&1 >&3 3>&- | grep bad 3>&- # Close fd 3 for 'grep' (but not 'ls'). # ^^^^ ^^^^ exec 3>&- # Now close it for the remainder of the script. # Thanks, S.C. For a more detailed introduction to I/O redirection see . The exec <filename command redirects stdin to a file. From that point on, all stdin comes from that file, rather than its normal source (usually keyboard input). This provides a method of reading a file line by line and possibly parsing each line of input using sed and/or awk. &redir1; Blocks of code, such as while, until, and for loops, even if/then test blocks can also incorporate redirection of stdin. Even a function may use this form of redirection (see ). The < operator at the the end of the code block accomplishes this. &redir2; &redir2a; &redir3; &redir4; We can modify the previous example to also redirect the output of the loop. &redir4a; &redir5; &namesdata; Redirecting the stdout of a code block has the effect of saving its output to a file. See . Here documents are a special case of redirected code blocks. Clever use of I/O redirection permits parsing and stitching together snippets of command output (see ). This permits generating report and log files. &logevents; << special character << A here document uses a special form of I/O redirection to feed a command script to an interactive program, such as ftp, telnet, or ex. Typically, the script consists of a command list to the program, delineated by a limit string. The special symbol << precedes the limit string. This has the effect of redirecting the output of a file into the program, similar to interactive-program < command-file, where command-file contains command #1 command #2 ... The here document alternative looks like this: #!/bin/bash interactive-program <<LimitString command #1 command #2 ... LimitString Choose a limit string sufficiently unusual that it will not occur anywhere in the command list and confuse matters. Note that here documents may sometimes be used to good effect with non-interactive utilities and commands. &ex69; The above script could just as effectively have been implemented with ex, rather than vi. Here documents containing a list of ex commands are common enough to form their own category, known as ex scripts. &ex70; &ex71; The - option to mark a here document limit string (<<-LimitString) suppresses tabs (but not spaces) in the output. This may be useful in making a script more readable. &ex71a; A here document supports parameter and command substitution. It is therefore possible to pass different parameters to the body of the here document, changing its output accordingly. &ex71b; Quoting or escaping the limit string at the head of a here document disables parameter substitution within its body. This has very limited usefulness. &ex71c; This is a useful script containing a here document with parameter substitution. &ex72; It is possible to use : as a dummy command accepting output from a here document. This, in effect, creates an anonymous here document. #!/bin/bash : <<TESTVARIABLES ${HOSTNAME?}${USER?}${MAIL?} # Print error message if one of the variables not set. TESTVARIABLES exit 0 Here documents create temporary files, but these files are deleted after opening and are not accessible to any other process. bash$ bash -c 'lsof -a -p $$ -d0' << EOF > EOF lsof 1213 bozo 0r REG 3,5 0 30386 /tmp/t1213-0-sh (deleted) Some utilities will not work inside a here document. For those tasks too complex for a here document, consider using the expect scripting language, which is specifically tailored for feeding input into interactive programs.

This bizarre little intermission gives the reader a chance to relax and maybe laugh a bit. Fellow Linux user, greetings! You are reading something which will bring you luck and good fortune. Just e-mail a copy of this document to 10 of your friends. Before you make the copies, send a 100-line Bash script to the first person on the list given at the bottom of this letter. Then delete their name and add yours to the bottom of the list. Don't break the chain! Make the copies within 48 hours. Wilfred P. of Brooklyn failed to send out his ten copies and woke the next morning to find his job description changed to "COBOL programmer." Howard L. of Newport News sent out his ten copies and within a month had enough hardware to build a 100-node Beowulf cluster dedicated to playing xbill. Amelia V. of Chicago laughed at this letter and broke the chain. Shortly thereafter, a fire broke out in her terminal and she now spends her days writing documentation for MS Windows. Don't break the chain! Send out your ten copies today! Courtesy 'NIX "fortune cookies", with some alterations and many apologies

To fully utilize the power of shell scripting, you need to master Regular Expressions. Certain commands and utilities commonly used in scripts, such as expr, sed and awk interpret and use REs. An expression is a string of characters. Those characters that have an interpretation above and beyond their literal meaning are called metacharacters. A quote symbol, for example, may denote speech by a person, ditto, or a meta-meaning for the symbols that follow. Regular Expressions are sets of characters and/or metacharacters that UNIX endows with special features. The simplest type of Regular Expression is a character string that retains its literal meaning, not containing any metacharacters. The main uses for Regular Expressions (REs) are text searches and string manipulation. An RE matches a single character or a set of characters (a substring or an entire string). * special character * The asterisk * matches any number of repeats of the character string or RE preceding it, including zero. 1133* matches 11 + one or more 3's + possibly other characters: 113, 1133, 111312, and so forth. . special character . The dot . matches any one character, except a newline. Since sed, awk, and grep process single lines, there will usually not be a newline to match. In those cases where there is a newline in a multiple line expression, the dot will match the newline. #!/bin/bash sed -e 'N;s/.*/[&]/' << EOF # Here Document line1 line2 EOF # OUTPUT: # [line1 # line2] echo awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' << EOF line 1 line 2 EOF # OUTPUT: # line # 1 # Thanks, S.C. exit 0 13. matches 13 + at least one of any character (including a space): 1133, 11333, but not 13 (additional character missing). ^ special character ^ The caret ^ matches the beginning of a line, but sometimes, depending on context, negates the meaning of a set of characters in an RE. $ special character $ The dollar sign $ at the end of an RE matches the end of a line. ^$ matches blank lines. [...] special character [...] Brackets [...] enclose a set of characters to match in a single RE. [xyz] matches the characters x, y, or z. [c-n] matches any of the characters in the range c to n. [B-Pk-y] matches any of the characters in the ranges B to P and k to y. [a-z0-9] matches any lowercase letter or any digit. [^b-d] matches all characters except those in the range b to d. This is an instance of ^ negating or inverting the meaning of the following RE (taking on a role similar to ! in a different context). Combined sequences of bracketed characters match common word patterns. [Yy][Ee][Ss] matches yes, Yes, YES, yEs, and so forth. [0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9] matches any Social Security number. \ special character \ The backslash \ escapes a special character, which means that character gets interpreted literally. A \$ reverts back to its literal meaning of $, rather than its RE meaning of end-of-line. Likewise a \\ has the literal meaning of \. Used in egrep, awk, and Perl ? special character ? The question mark ? matches zero or one of the previous RE. It is generally used for matching single characters. + special character + The plus + matches one or more of the previous RE. It serves a role similar to the *, but does not match zero occurrences. # GNU versions of sed and awk can use "+", # but it needs to be escaped. echo a111b | sed -ne '/a1\+b/p' echo a111b | grep 'a1\+b' echo a111b | gawk '/a1+b/' # All of above are equivalent. # Thanks, S.C. \{ \} special character \{ \} Escaped curly brackets \{ \} indicate the number of occurrences of a preceding RE to match. It is necessary to escape the curly brackets since they have only their literal character meaning otherwise. This usage is technically not part of the basic RE set. [0-9]\{5\} matches exactly five digits (characters in the range of 0 to 9). Curly brackets are not available as an RE in the classic version of awk. However, gawk has the --re-interval option that permits them (without being escaped). bash$ echo 2222 | gawk --re-interval '/2{3}/' 2222 () special character () Parentheses ( ) enclose groups of REs. They are especially useful with the following | operator. | special character | The | or RE operator matches any of a set of alternate characters. bash$ egrep 're(a|e)d' misc.txt People who read seem to be better informed than those who do not. The clarinet produces sound by the vibration of its reed. [:class:] [: special character :] This is an alternate method of specifying a range of characters to match. alnum character range alphabetic numeric [:alnum:] matches alphabetic or numeric characters. This is equivalent to [A-Za-z0-9]. alpha character range alphabetic [:alpha:] matches alphabetic characters. This is equivalent to [A-Za-z]. blank character range space tab [:blank:] matches a space or a tab. cntrl character range control [:cntrl:] matches control characters. digit character range decimal digit [:digit:] matches (decimal) digits. This is equivalent to [0-9]. graph character range graph [:graph:] (graphic printable characters). Matches characters in the range of ASCII 33 - 126. This is the same as [:print:], below, but excluding the space character. lower character range lowercase [:lower:] matches lowercase alphabetic characters. This is equivalent to [a-z]. print character range printable [:print:] (printable characters). Matches characters in the range of ASCII 32 - 126. This is the same as [:graph:], above, but adding the space character. space character range whitespace [:space:] matches whitespace characters (space and horizontal tab). upper character range uppercase [:upper:] matches uppercase alphabetic characters. This is equivalent to [A-Z]. xdigit character range hexadecimal [:xdigit:] matches hexadecimal digits. This is equivalent to [0-9A-Fa-f]. POSIX character classes generally require quoting or double brackets ([[ ]]). bash$ grep [[:digit:]] test.file abc=723 These character classes may even be used with globbing, to a limited extent. bash$ ls -l ?[[:digit:]][[:digit:]]? -rw-rw-r-- 1 bozo bozo 0 Aug 21 14:47 a33b To see POSIX character classes used in scripts, refer to and . Sed, awk, and Perl, used as filters in scripts, take REs as arguments when "sifting" or transforming files or I/O streams. See and for illustrations of this. "Sed & Awk", by Dougherty and Robbins gives a very complete and lucid treatment of REs (see the ). Bash itself cannot recognize Regular Expressions. In scripts, commands and utilities, such as sed and awk, interpret RE's. Bash does carry out filename expansion, a process known as globbing, but this does not use the standard RE set. Instead, globbing recognizes and expands wildcards. Globbing interprets the standard wildcard characters, * and ?, character lists in square brackets, and certain other special characters (such as ^ for negating the sense of a match). There are some important limitations on wildcard characters in globbing, however. Strings containing * will not match filenames that start with a dot, as, for example, .bashrc. Filename expansion can match dotfiles, but only if the pattern explicitly includes the dot. ~/[.]bashrc # Will not expand to ~/.bashrc ~/?bashrc # Neither will this. # Wild cards and metacharacters will not expand to a dot in globbing. ~/.[b]ashrc # Will expand to ~./bashrc ~/.ba?hrc # Likewise. ~/.bashr* # Likewise. # Setting the "dotglob" option turns this off. # Thanks, S.C. Likewise, the ? has a different meaning in globbing than as part of an RE. bash$ ls -l total 2 -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1 -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1 -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1 -rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh -rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt bash$ ls -l t?.sh -rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh bash$ ls -l [ab]* -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1 -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1 bash$ ls -l [a-c]* -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1 -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1 -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1 bash$ ls -l [^ab]* -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1 -rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh -rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt bash$ ls -l {b*,c*,*est*} -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1 -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1 -rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt bash$ echo * a.1 b.1 c.1 t2.sh test1.txt bash$ echo t* t2.sh test1.txt Even an echo command performs wildcard expansion on filenames. See also . Running a shell script launches another instance of the command processor. Just as your commands are interpreted at the command line prompt, similarly does a script batch process a list of commands in a file. Each shell script running is, in effect, a subprocess of the parent shell, the one that gives you the prompt at the console or in an xterm window. A shell script can also launch subprocesses. These subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously. ( command1; command2; command3; ... ) A command list embedded between parentheses runs as a subshell. Variables in a subshell are not visible outside the block of code in the subshell. They are not accessible to the parent process, to the shell that launched the subshell. These are, in effect, local variables. &subshell; See also . + Directory changes made in a subshell do not carry over to the parent shell. &allprofs; A subshell may be used to set up a dedicated environment for a command group. COMMAND1 COMMAND2 COMMAND3 ( IFS=: PATH=/bin unset TERMINFO set -C shift 5 COMMAND4 COMMAND5 exit 3 # Only exits the subshell. ) # The parent shell has not been affected, and the environment is preserved. COMMAND6 COMMAND7 One application of this is testing whether a variable is defined. if (set -u; : $variable) 2> /dev/null then echo "Variable is set." fi # Could also be written [[ ${variable-x} != x || ${variable-y} != y ]] # or [[ ${variable-x} != x$variable ]] # or [[ ${variable+x} = x ]]) Another application is checking for a lock file: if (set -C; : > lock_file) 2> /dev/null then echo "Another user is already running that script." exit 65 fi # Thanks, S.C. Processes may execute in parallel within different subshells. This permits breaking a complex task into subcomponents processed concurrently. (cat list1 list2 list3 | sort | uniq > list123) & (cat list4 list5 list6 | sort | uniq > list456) & # Merges and sorts both sets of lists simultaneously. # Running in background ensures parallel execution. # # Same effect as # cat list1 list2 list3 | sort | uniq > list123 & # cat list4 list5 list6 | sort | uniq > list456 & wait # Don't execute the next command until subshells finish. diff list123 list456 Redirecting I/O to a subshell uses the | pipe operator, as in ls -al | (command). A command block between curly braces does not launch a subshell. { command1; command2; command3; ... } Running a script or portion of a script in restricted mode disables certain commands that would otherwise be available. This is a security measure intended to limit the privileges of the script user and to minimize possible damage from running the script. Using cd to change the working directory. Changing the values of the $PATH, $SHELL, $BASH_ENV, or $ENV environmental variables. Reading or changing the $SHELLOPTS, shell environmental options. Output redirection. Invoking commands containing one or more /'s. Invoking exec to substitute a different process for the shell. Various other commands that would enable monkeying with or attempting to subvert the script for an unintended purpose. Getting out of restricted mode within the script. &restricted; Process substitution is the counterpart to command substitution. Command substitution sets a variable to the result of a command, as in dir_contents=`ls -al` or xref=$( grep word datafile). Process substitution feeds the output of a process to another process (in other words, it sends the results of a command to another command). command within parentheses >(command) <(command) These initiate process substitution. This uses /dev/fd/<n> files to send the results of the process within parentheses to another process. This has the same effect as a named pipe (temp file), and, in fact, named pipes were at one time used in process substitution. There is no space between the the < or > and the parentheses. Space there would give an error message. bash$ echo >(true) /dev/fd/63 bash$ echo <(true) /dev/fd/63 Bash creates a pipe with two file descriptors, --fIn and fOut--. The stdin of true connects to fOut (dup2(fOut, 0)), then Bash passes a /dev/fd/fIn argument to echo. On systems lacking /dev/fd/<n> files, Bash may use temporary files. (Thanks, S.C.) cat <(ls -l) # Same as ls -l | cat sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin) # Lists all the files in the 3 main 'bin' directories, and sorts by filename. # Note that three (count 'em) distinct commands are fed to 'sort'. diff <(command1) <(command2) # Gives difference in command output. tar cf >(bzip2 -c > file.tar.bz2) dir # Calls "tar cf /dev/fd/?? dir", and "bzip2 -c > file.tar.bz2". # # Because of the /dev/fd/<n> system feature, # the pipe between both commands does not need to be named. # # This can be emulated. # bzip2 -c < pipe > file.tar.bz2& tar cf pipe dir rm pipe # or exec 3>&1 tar cf /dev/fd/4 dir 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&- exec 3>&- # Thanks, S.C. A reader of this document sent in the following interesting example of process substitution. # Script fragment taken from SuSE distribution: while read des what mask iface; do # Some commands ... done < <(route -n) # To test it, let's make it do something. while read des what mask iface; do echo $des $what $mask $iface done < <(route -n) # Output: # Kernel IP routing table # Destination Gateway Genmask Flags Metric Ref Use Iface # 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo # As S.C. points out, an easier-to-understand equivalent is: route -n | while read des what mask iface; do # Variables set from output of pipe. echo $des $what $mask $iface done # Same output as above. Like real programming languages, Bash has functions, though in a somewhat limited implementation. A function is a subroutine, a code block that implements a set of operations, a black box that performs a specified task. Wherever there is repetitive code, when a task repeats with only slight variations, then consider using a function. function function_name { command } or function_name () { command } This second form will cheer the hearts of C programmers (and is more portable). As in C, the function's opening bracket may optionally appear on the second line. function_name () { command } Functions are called, triggered, simply by invoking their names. &ex59; The function definition must precede the first call to it. There is no method of declaring the function, as, for example, in C. # f1 # Will give an error message, since function "f1" not yet defined. # However... f1 () { echo "Calling function \"f2\" from within function \"f1\"." f2 } f2 () { echo "Function \"f2\"." } f1 # Function "f2" is not actually called until this point, # although it is referenced before its definition. # This is permissable. # Thanks, S.C. It is even possible to nest a function within another function, although this is not very useful. f1 () { f2 () # nested { echo "Function \"f2\", inside \"f1\"." } } # f2 # Gives an error message. f1 # Does nothing, since calling "f1" does not automatically call "f2". f2 # Now, it's all right to call "f2", # since its definition has been made visible by calling "f1". # Thanks, S.C. Function declarations can appear in unlikely places, even where a command would otherwise go. ls -l | foo() { echo "foo"; } # Permissable, but useless. if [ "$USER" = bozo ] then bozo_greet () # Function definition embedded in an if/then construct. { echo "Hello, Bozo." } fi bozo_greet # Works only for Bozo, and other users get an error. # Something like this might be useful in some contexts. NO_EXIT=1 # Will enable function definition below. [[ $NO_EXIT -eq 1 ]] && exit() { true; } # Function definition in an "and-list". # If $NO_EXIT is 1, declares "exit ()". # This disables the "exit" builtin by aliasing it to "true". exit # Invokes "exit ()" function, not "exit" builtin. # Thanks, S.C. Functions may process arguments passed to them and return an exit status to the script for further processing. function_name $arg1 $arg2 The function refers to the passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth. &ex60; The shift command works on arguments passed to functions (see ). In contrast to certain other programming languages, shell scripts normally pass only value parameters to functions. Indirect variable references (see ) provide a clumsy sort of mechanism for passing variable pointers to functions. &refparams; Variable names (which are actually pointers), if passed as parameters to functions, will be treated as string literals and cannot be dereferenced. Functions interpret their arguments literally. exit status Functions return a value, called an exit status. The exit status may be explicitly specified by a return statement, otherwise it is the exit status of the last command in the function (0 if successful, and a non-zero error code if not). This exit status may be used in the script by referencing it as $?. This mechanism effectively permits script functions to have a return value similar to C functions. return return command return Terminates a function. A return command The return command is a Bash builtin. optionally takes an integer argument, which is returned to the calling script as the exit status of the function, and this exit status is assigned to the variable $?. &max; For a function to return a string or array, use a dedicated variable. count_lines_in_etc_passwd() { [[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd)) # If /etc/passwd is readable, set REPLY to line count. # Returns both a parameter value and status information. } if count_lines_in_etc_passwd then echo "There are $REPLY lines in /etc/passwd." else echo "Cannot count lines in /etc/passwd." fi # Thanks, S.C. &ex61; See also . The largest positive integer a function can return is 256. The return command is closely tied to the concept of exit status, which accounts for this particular limitation. Fortunately, there are various workarounds for those situations requiring a large integer return value from a function. &returntest; As we have seen, a function can return a large negative value. This also permits returning large positive integer, using a bit of trickery. An alternate method of accomplishing this is to simply assign the return value to a global variable. Return_Val= # Global variable to hold oversize return value of function. alt_return_test () { fvar=$1 Return_Val=$fvar return # Returns 0 (success). } alt_return_test 1 echo $? # 0 echo "return value = $Return_Val" # 1 alt_return_test 256 echo "return value = $Return_Val" # 256 alt_return_test 257 echo "return value = $Return_Val" # 257 alt_return_test 25701 echo "return value = $Return_Val" #25701 &max2; See also . Exercise: Using what we have just learned, extend the previous Roman numerals example to accept arbitrarily large input. Redirecting the stdin of a function redirection stdin A function is essentially a code block, which means its stdin can be redirected (as in ). &realname; There is an alternative, and perhaps less confusing method of redirecting a function's stdin. This involves redirecting the stdin to an embedded bracketed code block within the function. # Instead of: Function () { ... } < file # Try this: Function () { { ... } < file } # Similarly, Function () # This works. { { echo $* } | tr a b } Function () # This doesn't work. { echo $* } | tr a b # A nested code block is mandatory here. # Thanks, S.C. local variables variable local A variable declared as local is one that is visible only within the block of code in which it appears. It has local scope. In a function, a local variable has meaning only within that function block. &ex62; Before a function is called, all variables declared within the function are invisible outside the body of the function, not just those explicitly declared as local. #!/bin/bash func () { global_var=37 # Visible only within the function block #+ before the function has been called. } # END OF FUNCTION echo "global_var = $global_var" # global_var = # Function "func" has not yet been called, #+ so $global_var is not visible here. func echo "global_var = $global_var" # global_var = 37 # Has been set by function call. Local variables permit recursion, Herbert Mayer defines recursion as ...expressing an algorithm by using a simpler version of that same algorithm... A recursive function is one that calls itself. but this practice generally involves much computational overhead and is definitely not recommended in a shell script. Too many levels of recursion may crash a script with a segfault. #!/bin/bash recursive_function () { (( $1 < $2 )) && f $(( $1 + 1 )) $2; # As long as 1st parameter is less than 2nd, #+ increment 1st and recurse. } recursive_function 1 50000 # Recurse 50,000 levels! # Segfaults, of course. # Recursion this deep might cause even a C program to segfault, #+ by using up all the memory allotted to the stack. # Thanks, S.C. exit 0 # This script will not exit normally. &ex63; See also for an example of recursion in a script. Be aware that recursion is resource-intensive and executes slowly, and is therefore generally not appropriate to use in a script. alias A Bash alias is essentially nothing more than a keyboard shortcut, an abbreviation, a means of avoiding typing a long command sequence. If, for example, we include alias lm="ls -l | more" in the ~/.bashrc file, then each lm typed at the command line will automatically be replaced by a ls -l | more. This can save a great deal of typing at the command line and avoid having to remember complex combinations of commands and options. Setting alias rm="rm -i" (interactive mode delete) may save a good deal of grief, since it can prevent inadvertently losing important files. In a script, aliases have very limited usefulness. It would be quite nice if aliases could assume some of the functionality of the C preprocessor, such as macro expansion, but unfortunately Bash does not expand arguments within the alias body. However, aliases do seem to expand positional parameters. Moreover, a script fails to expand an alias itself within compound constructs, such as if/then statements, loops, and functions. An added limitation is that an alias will not expand recursively. Almost invariably, whatever we would like an alias to do could be accomplished much more effectively with a function. &al; The unalias command removes a previously set alias. &unal; bash$ ./unalias.sh total 6 drwxrwxr-x 2 bozo bozo 3072 Feb 6 14:04 . drwxr-xr-x 40 bozo bozo 2048 Feb 6 14:04 .. -rwxr-xr-x 1 bozo bozo 199 Feb 6 14:04 unalias.sh ./unalias.sh: llm: command not found && special character && AND list || special character || OR list The and list and or list constructs provide a means of processing a number of commands consecutively. These can effectively replace complex nested if/then or even case statements. and list command-1 && command-2 && command-3 && ... command-n Each command executes in turn provided that the previous command has given a return value of true (zero). At the first false (non-zero) return, the command chain terminates (the first command returning false is the last one to execute). &ex64; &andlist2; or list command-1 || command-2 || command-3 || ... command-n Each command executes in turn for as long as the previous command returns false. At the first true return, the command chain terminates (the first command returning true is the last one to execute). This is obviously the inverse of the and list. &ex65; If the first command in an or list returns true, it will execute. The exit status of an and list or an or list is the exit status of the last command executed. Clever combinations of and and or lists are possible, but the logic may easily become convoluted and require extensive debugging. false && true || echo false # false # Same result as ( false && true ) || echo false # false # But *not* false && ( true || echo false ) # (nothing echoed) # Note left-to-right grouping and evaluation of statements, # since the logic operators "&&" and "||" have equal precedence. # It's best to avoid such complexities, unless you know what you're doing. # Thanks, S.C. See for an illustration of using an and / or list to test variables. Newer versions of Bash support one-dimensional arrays. Array elements may be initialized with the variable[xx] notation. Alternately, a script may introduce the entire array by an explicit declare -a variable statement. To dereference (find the contents of) an array element, use curly bracket notation, that is, ${variable[xx]}. &ex66; Arrays variables have a syntax all their own, and even standard Bash commands and operators have special options adapted for array use. array=( zero one two three four five ) echo ${array[0]} # zero echo ${array:0} # zero # Parameter expansion of first element. echo ${array:1} # ero # Parameter expansion of first element, #+ starting at position #1 (2nd character). echo ${#array} # 4 # Length of first element of array. In an array context, some Bash builtins have a slightly altered meaning. For example, unset deletes array elements, or even an entire array. &ex67; As seen in the previous example, either ${array_name[@]} or ${array_name[*]} refers to all the elements of the array. Similarly, to get a count of the number of elements in an array, use either ${#array_name[@]} or ${#array_name[*]}. ${#array_name} is the length (number of characters) of ${array_name[0]}, the first element of the array. &emptyarray; The relationship of ${array_name[@]} and ${array_name[*]} is analogous to that between $@ and $*. This powerful array notation has a number of uses. # Copying an array. array2=( "${array1[@]}" ) # or array2="${array1[@]}" # Adding an element to an array. array=( "${array[@]}" "new element" ) # or array[${#array[*]}]="new element" # Thanks, S.C. The array=( element1 element2 ... elementN ) initialization operation, with the help of command substitution, makes it possible to load the contents of a text file into an array. #!/bin/bash filename=sample_file # cat sample_file # # 1 a b c # 2 d e fg declare -a array1 array1=( `cat "$filename" | tr '\n' ' '`) # Loads contents # of $filename into array1. # list file to stdout. # change linefeeds in file to spaces. echo ${array1[@]} # List the array. # 1 a b c 2 d e fg # # Each whitespace-separated "word" in the file #+ has been assigned to an element of the array. element_count=${#array1[*]} echo $element_count # 8 Arrays permit deploying old familiar algorithms as shell scripts. Whether this is necessarily a good idea is left to the reader to decide. &bubble; -- Arrays enable implementing a shell script version of the Sieve of Erastosthenes. Of course, a resource-intensive application of this nature should really be written in a compiled language, such as C. It runs excruciatingly slowly as a script. &ex68; Compare this array-based prime number generator with with an alternative that does not use arrays, . -- Arrays lend themselves, to some extent, to emulating data structures for which Bash has no native support. &stackex; -- Fancy manipulation of array subscripts may require intermediate variables. For projects involving this, again consider using a more powerful programming language, such as Perl or C. &qfunction; -- Bash supports only one-dimensional arrays, however a little trickery permits simulating multi-dimensional ones. &twodim; A two-dimensional array is essentially equivalent to a one-dimensional one, but with additional addressing modes for referencing and manipulating the individual elements by row and column position. For an even more elaborate example of simulating a two-dimensional array, see . These files contain the aliases and environmental variables made available to Bash running as a user shell and to all Bash scripts invoked after system initialization. /etc/profile systemwide defaults, mostly setting the environment (all Bourne-type shells, not just Bash This does not apply to csh, tcsh, and other shells not related to or descended from the classic Bourne shell (sh).) /etc/bashrc systemwide functions and and aliases for Bash $HOME/.bash_profile user-specific Bash environmental default settings, found in each user's home directory (the local counterpart to /etc/profile) $HOME/.bashrc user-specific Bash init file, found in each user's home directory (the local counterpart to /etc/bashrc). Only interactive shells and user scripts read this file. See for a sample .bashrc file. $HOME/.bash_logout user-specific instruction file, found in each user's home directory. Upon exit from a login (Bash) shell, the commands in this file execute. A Linux or UNIX machine typically has two special-purpose directories, /dev and /proc. The /dev directory contains entries for the physical devices that may or may not be present in the hardware. The entries in /dev provide mount points for physical and virtual devices. These entries use very little drive space. Some devices, such as /dev/null, /dev/zero, and /dev/urandom are virtual. They are not actual physical devices and exist only in software. The hard drive partitions containing the mounted filesystem(s) have entries in /dev, as a simple df shows. bash$ df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda6 495876 222748 247527 48% / /dev/hda1 50755 3887 44248 9% /boot /dev/hda8 367013 13262 334803 4% /home /dev/hda5 1714416 1123624 503704 70% /usr Among other things, the /dev directory also contains loopback devices, such as /dev/loop0. A loopback device is a gimmick allows an ordinary file to be accessed as if it were a block device. A block device reads and/or writes data in chunks, or blocks, in contrast to a character device, which acesses data in character units. Examples of block devices are a hard drive and CD ROM drive. An example of a character device is a keyboard. This enables mounting an entire filesystem within a single large file. See and . A few of the pseudo-devices in /dev have other specialized uses, such as /dev/null, /dev/zero and /dev/urandom. The /proc directory is actually a pseudo-filesystem. The files in the /proc directory mirror currently running system and kernel processes and contain information and statistics about them. bash$ cat /proc/devices Character devices: 1 mem 2 pty 3 ttyp 4 ttyS 5 cua 7 vcs 10 misc 14 sound 29 fb 36 netlink 128 ptm 136 pts 162 raw 254 pcmcia Block devices: 1 ramdisk 2 fd 3 ide0 9 md bash$ cat /proc/interrupts CPU0 0: 84505 XT-PIC timer 1: 3375 XT-PIC keyboard 2: 0 XT-PIC cascade 5: 1 XT-PIC soundblaster 8: 1 XT-PIC rtc 12: 4231 XT-PIC PS/2 Mouse 14: 109373 XT-PIC ide0 NMI: 0 ERR: 0 bash$ cat /proc/partitions major minor #blocks name rio rmerge rsect ruse wio wmerge wsect wuse running use aveq 3 0 3007872 hda 4472 22260 114520 94240 3551 18703 50384 549710 0 111550 644030 3 1 52416 hda1 27 395 844 960 4 2 14 180 0 800 1140 3 2 1 hda2 0 0 0 0 0 0 0 0 0 0 0 3 4 165280 hda4 10 0 20 210 0 0 0 0 0 210 210 ... bash$ cat /proc/loadavg 0.13 0.42 0.27 2/44 1119 Shell scripts may extract data from certain of the files in /proc. Certain system commands, such as procinfo, free, vmstat, lsdev, and uptime do this as well. kernel_version=$( awk '{ print $3 }' /proc/version ) CPU=$( awk '/model name/ {print $4}' < /proc/cpuinfo ) if [ $CPU = Pentium ] then run_some_commands ... else run_different_commands ... fi The /proc directory contains subdirectories with unusual numerical names. Every one of these names maps to the process ID of a currently running process. Within each of these subdirectories, there are a number of files that hold useful information about the corresponding process. The stat and status files keep running statistics on the process, the cmdline file holds the command-line arguments the process was invoked with, and the exe file is a symbolic link to the complete path name of the invoking process. There are a few more such files, but these seem to be the most interesting from a scripting standpoint. &pidid; &constat; In general, it is dangerous to write to the files in /proc, as this can corrupt the filesystem or crash the machine. Uses of /dev/null Think of /dev/null as a black hole. It is the nearest equivalent to a write-only file. Everything written to it disappears forever. Attempts to read or output from it result in nothing. Nevertheless, /dev/null can be quite useful from both the command line and in scripts. Suppressing stdout or stderr (from ): rm $badname 2>/dev/null # So error messages [stderr] deep-sixed. Deleting contents of a file, but preserving the file itself, with all attendant permissions (from and ): cat /dev/null > /var/log/messages # : > /var/log/messages has same effect, but does not spawn a new process. cat /dev/null > /var/log/wtmp Automatically emptying the contents of a logfile (especially good for dealing with those nasty cookies sent by Web commercial sites): if [ -f ~/.netscape/cookies ] # Remove, if exists. then rm -f ~/.netscape/cookies fi ln -s /dev/null ~/.netscape/cookies # All cookies now get sent to a black hole, rather than saved to disk. Uses of /dev/zero Like /dev/null, /dev/zero is a pseudo file, but it actually contains nulls (numerical zeros, not the ASCII kind). Output written to it disappears, and it is fairly difficult to actually read the nulls in /dev/zero, though it can be done with od or a hex editor. The chief use for /dev/zero is in creating an initialized dummy file of specified length intended as a temporary swap file. &ex73; Another application of /dev/zero is to zero out a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device (see ) or securely deleting a file (see ). &ramdisk; The Bash shell contains no debugger, nor even any debugging-specific commands or constructs. Syntax errors or outright typos in the script generate cryptic error messages that are often of no help in debugging a non-functional script. &ex74; Output from script: ./ex74.sh: [37: command not found What's wrong with the above script (hint: after the if)? What if the script executes, but does not work as expected? This is the all too familiar logic error. &ex75; Try to find out what's wrong with by uncommenting the echo "$badname" line. Echo statements are useful for seeing whether what you expect is actually what you get. In this particular case, rm "$badname" will not give the desired results because $badname should not be quoted. Placing it in quotes ensures that rm has only one argument (it will match only one filename). A partial fix is to remove to quotes from $badname and to reset $IFS to contain only a newline, IFS=$'\n'. However, there are simpler ways of going about it. # Correct methods of deleting filenames containing spaces. rm *\ * rm *" "* rm *' '* # Thank you. S.C. Summarizing the symptoms of a buggy script, It bombs with an error message syntax error, or It runs, but does not work as expected (logic error) It runs, works as expected, but has nasty side effects (logic bomb). Tools for debugging non-working scripts include echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of what is going on. using the tee filter to check processes or data flows at critical points. setting option flags -n -v -x sh -n scriptname checks for syntax errors without actually running the script. This is the equivalent of inserting set -n or set -o noexec into the script. Note that certain types of syntax errors can slip past this check. sh -v scriptname echoes each command before executing it. This is the equivalent of inserting set -v or set -o verbose in the script. The -n and -v flags work well together. sh -nv scriptname gives a verbose syntax check. sh -x scriptname echoes the result each command, but in an abbreviated manner. This is the equivalent of inserting set -x or set -o xtrace in the script. Inserting set -u or set -o nounset in the script runs it, but gives an unbound variable error message at each attempt to use an undeclared variable. Using an assert function to test a variable or condition at critical points in a script. (This is an idea borrowed from C.) &assert; trapping at exit. The exit command in a script triggers a signal 0, terminating the process, that is, the script itself. By convention, signal 0 is assigned to exit. It is often useful to trap the exit, forcing a printout of variables, for example. The trap must be the first command in the script. trap Specifies an action on receipt of a signal; also useful for debugging. A signal is simply a message sent to a process, either by the kernel or another process, telling it to take some specified action (usually to terminate). For example, hitting a ControlC, sends a user interrupt, an INT signal, to a running program. trap '' 2 # Ignore interrupt 2 (Control-C), with no action specified. trap 'echo "Control-C disabled."' 2 # Message when Control-C pressed. &ex76; &online; The DEBUG argument to trap causes a specified action to execute after every command in a script. This permits tracing variables, for example. &vartrace; trap '' SIGNAL (two adjacent apostrophes) disables SIGNAL for the remainder of the script. trap SIGNAL restores the functioning of SIGNAL once more. This is useful to protect a critical portion of a script from an undesirable interrupt. trap '' 2 # Signal 2 is Control-C, now disabled. command command command trap 2 # Reenables Control-C Options are settings that change shell and/or script behavior. The set command enables options within a script. At the point in the script where you want the options to take effect, use set -o option-name or, in short form, set -option-abbrev. These two forms are equivalent. #!/bin/bash set -o verbose # Echoes all commands before executing. #!/bin/bash set -v # Exact same effect as above. To disable an option within a script, use set +o option-name or set +option-abbrev. #!/bin/bash set -o verbose # Command echoing on. command ... command set +o verbose # Command echoing off. command # Not echoed. set -v # Command echoing on. command ... command set +v # Command echoing off. command exit 0 An alternate method of enabling options in a script is to specify them immediately following the #! script header. #!/bin/bash -x # # Body of script follows. It is also possible to enable script options from the command line. Some options that will not work with set are available this way. Among these are -i, force script to run interactive. bash -v script-name bash -o verbose script-name The following is a listing of some useful options. They may be specified in either abbreviated form or by complete name. Abbreviation Name Effect -C noclobber Prevent overwriting of files by redirection (may be overridden by >|) -D (none) List double-quoted strings prefixed by $, but do not execute commands in script -a allexport Export all defined variables -b notify Notify when jobs running in background terminate (not of much use in a script) -c ... (none) Read commands from ... -f noglob Filename expansion (globbing) disabled -i interactive Script runs in interactive mode -p privileged Script runs as suid (caution!) -r restricted Script runs in restricted mode (see ). -u nounset Attempt to use undefined variable outputs error message, and forces an exit -v verbose Print each command to stdout before executing it -x xtrace Similar to -v, but expands commands -e errexit Abort script at first error (when a command exits with non-zero status) -n noexec Read commands in script, but do not execute them (syntax check) -s stdin Read commands from stdin -t (none) Exit after first command - (none) End of options flag. All other arguments are positional parameters. -- (none) Unset positional parameters. If arguments given (-- arg1 arg2), positional parameters set to arguments. Puccini Turandot: Gli enigmi sono tre, la morte una! Caleph: No, no! Gli enigmi sono tre, una la vita! Assigning reserved words or characters to variable names. case=value0 # Causes problems. 23skidoo=value1 # Also problems. # Variable names starting with a digit are reserved by the shell. # Try _23skidoo=value1. Starting variables with an underscore is o.k. # However... using just the underscore will not work. _=25 echo $_ # $_ is a special variable set to last arg of last command. xyz((!*=value2 # Causes severe problems. Using a hyphen or other reserved characters in a variable name. var-1=23 # Use 'var_1' instead. Using the same name for a variable and a function. This can make a script difficult to understand. do_something () { echo "This function does something with \"$1\"." } do_something=do_something do_something do_something # All this is legal, but highly confusing. Using whitespace inappropriately (in contrast to other programming languages, Bash can be quite finicky about whitespace). var1 = 23 # 'var1=23' is correct. # On line above, Bash attempts to execute command "var1" # with the arguments "=" and "23". let c = $a - $b # 'let c=$a-$b' or 'let "c = $a - $b"' are correct. if [ $a -le 5] # if [ $a -le 5 ] is correct. # if [ "$a" -le 5 ] is even better. # [[ $a -le 5 ]] also works. Assuming uninitialized variables (variables before a value is assigned to them) are zeroed out. An uninitialized variable has a value of null, not zero. Mixing up = and -eq in a test. Remember, = is for comparing literal variables and -eq for integers. if [ "$a" = 273 ] # Is $a an integer or string? if [ "$a" -eq 273 ] # If $a is an integer. # Sometimes you can mix up -eq and = without adverse consequences. # However... a=273.0 # Not an integer. if [ "$a" = 273 ] then echo "Comparison works." else echo "Comparison does not work." fi # Comparison does not work. # Same with a=" 273" and a="0273". # Likewise, problems trying to use "-eq" with non-integer values. if [ "$a" -eq 273.0 ] then echo "a = $a' fi # Aborts with an error message. # test.sh: [: 273.0: integer expression expected Mixing up integer and string comparison operators. #!/bin/bash # bad-op.sh number=1 while [ "$number" < 5 ] # Wrong! Should be while [ "number" -lt 5 ] do echo -n "$number " let "number += 1" done # Attempt to run this bombs with the error message: # bad-op.sh: 5: No such file or directory Sometimes variables within test brackets ([ ]) need to be quoted (double quotes). Failure to do so may cause unexpected behavior. See , , and . Commands issued from a script may fail to execute because the script owner lacks execute permission for them. If a user cannot invoke a command from the command line, then putting it into a script will likewise fail. Try changing the attributes of the command in question, perhaps even setting the suid bit (as root, of course). Attempting to use - as a redirection operator (which it is not) will usually result in an unpleasant surprise. command1 2> - | command2 # Trying to redirect error output of command1 into a pipe... # ...will not work. command1 2>& - | command2 # Also futile. Thanks, S.C. Using Bash version 2+ functionality may cause a bailout with error messages. Older Linux machines may have version 1.XX of Bash as the default installation. #!/bin/bash minimum_version=2 # Since Chet Ramey is constantly adding features to Bash, # you may set $minimum_version to 2.XX, or whatever is appropriate. E_BAD_VERSION=80 if [ "$BASH_VERSION" \< "$minimum_version" ] then echo "This script works only with Bash, version $minimum or greater." echo "Upgrade strongly recommended." exit $E_BAD_VERSION fi ... Using Bash-specific functionality in a Bourne shell script (#!/bin/sh) on a non-Linux machine may cause unexpected behavior. A Linux system usually aliases sh to bash, but this does not necessarily hold true for a generic UNIX machine. A script with DOS-type newlines (\r\n) will fail to execute, since #!/bin/bash\r\n is not recognized, not the same as the expected #!/bin/bash\n. The fix is to convert the script to UNIX-style newlines. A shell script headed by #!/bin/sh may not run in full Bash-compatibility mode. Some Bash-specific functions might be disabled. Scripts that need complete access to all the Bash-specific extensions should start with #!/bin/bash. A script may not export variables back to its parent process, the shell, or to the environment. Just as we learned in biology, a child process can inherit from a parent, but not vice versa. WHATEVER=/home/bozo export WHATEVER exit 0 bash$ echo $WHATEVER bash$ Sure enough, back at the command prompt, $WHATEVER remains unset. Setting and manipulating variables in a subshell, then attempting to use those same variables outside the scope of the subshell will result an unpleasant surprise. &subpit; Using suid commands in scripts is risky, as it may compromise system security. Setting the suid permission on a script has no effect. Using shell scripts for CGI programming may be problematic. Shell script variables are not typesafe, and this can cause undesirable behavior as far as CGI is concerned. Moreover, it is difficult to cracker-proof shell scripts. A.J. Lamb and H.W. Petrie Danger is near thee -- Beware, beware, beware, beware. Many brave hearts are asleep in the deep. So beware -- Beware. Get into the habit of writing shell scripts in a structured and systematic manner. Even on-the-fly and written on the back of an envelope scripts will benefit if you take a few minutes to plan and organize your thoughts before sitting down and coding. Herewith are a few stylistic guidelines. This is not intended as an Official Shell Scripting Stylesheet. Comment your code. This makes it easier for others to understand (and appreciate), and easier for you to maintain. PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}" # It made perfect sense when you wrote it last year, but now it's a complete mystery. # (From Antek Sawicki's "pw.sh" script.) Add descriptive headers to your scripts and functions. #!/bin/bash #************************************************# # xyz.sh # written by Bozo Bozeman # July 05, 2001 # Clean up project files. #************************************************# BADDIR=65 # No such directory. projectdir=/home/bozo/projects # Directory to clean up. #-------------------------------------------------------# # cleanup_pfiles () # Removes all files in designated directory. # Parameter: $target_directory # Returns: 0 on success, $BADDIR if something went wrong. #-------------------------------------------------------# cleanup_pfiles () { if [ ! -d "$1" ] # Test if target directory exists. then echo "$1 is not a directory." return $BADDIR fi rm -f "$1"/* return 0 # Success. } cleanup_pfiles $projectdir exit 0 Be sure to put the #!/bin/bash at the beginning of the first line of the script, preceding any comment headers. Avoid using magic numbers, In this context, magic numbers have an entirely different meaning than the magic numbers used to designate file types. that is, hard-wired literal constants. Use meaningful variable names instead. This makes the script easier to understand and permits making changes and updates without breaking the application. if [ -f /var/log/messages ] then ... fi # A year later, you decide to change the script to check /var/log/syslog. # It is now necessary to manually change the script, instance by instance, # and hope nothing breaks. # A better way: LOGFILE=/var/log/messages # Only line that needs to be changed. if [ -f "$LOGFILE" ] then ... fi Choose descriptive names for variables and functions. fl=`ls -al $dirname` # Cryptic. file_listing=`ls -al $dirname` # Better. MAXVAL=10 # All caps used for a script constant. while [ "$index" -le "$MAXVAL" ] ... E_NOTFOUND=75 # Uppercase for an errorcode, # and name begins with "E_". if [ ! -e "$filename" ] then echo "File $filename not found." exit $E_NOTFOUND fi MAIL_DIRECTORY=/var/spool/mail/bozo # Uppercase for an environmental variable. export MAIL_DIRECTORY GetAnswer () # Mixed case works well for a function. { prompt=$1 echo -n $prompt read answer return $answer } GetAnswer "What is your favorite number? " favorite_number=$? echo $favorite_number _uservariable=23 # Permissable, but not recommended. # It's better for user-defined variables not to start with an underscore. # Leave that for system variables. Use exit codes in a systematic and meaningful way. E_WRONG_ARGS=65 ... ... exit $E_WRONG_ARGS See also . Break complex scripts into simpler modules. Use functions where appropriate. See . Don't use a complex construct where a simpler one will do. COMMAND if [ $? -eq 0 ] ... # Redundant and non-intuitive. if COMMAND ... # More concise (if perhaps not quite as legible). Tom Duff Nobody really knows what the Bourne shell's grammar is. Even examination of the source code is little help. An interactive shell reads commands from user input on a tty. Among other things, such a shell reads startup files on activation, displays a prompt, and enables job control by default. The user can interact with the shell. A shell running a script is always a non-interactive shell. All the same, the script can still access its tty. It is even possible to emulate an interactive shell in a script. #!/bin/bash MY_PROMPT='$ ' while : do echo -n "$MY_PROMPT" read line eval "$line" done exit 0 # This example script, and much of the above explanation supplied by # Stephane Chazelas (thanks again). Let us consider an interactive script to be one that requires input from the user, usually with read statements (see ). Real life is actually a bit messier than that. For now, assume an interactive script is bound to a tty, a script that a user has invoked from the console or an xterm. Init and startup scripts are necessarily non-interactive, since they must run without human intervention. Many administrative and system maintenance scripts are likewise non-interactive. Unvarying repetitive tasks cry out for automation by non-interactive scripts. Non-interactive scripts can run in the background, but interactive ones hang, waiting for input that never comes. Handle that difficulty by having an expect script or embedded here document feed input to an interactive script running as a background job. In the simplest case, redirect a file to supply input to a read statement (read variable <file). These particular workarounds make possible general purpose scripts that run in either interactive or non-interactive modes. If a script needs to test whether it is running in an interactive shell, it is simply a matter of finding whether the prompt variable, $PS1 is set. (If the user is being prompted for input, then the script needs to display a prompt.) if [ -z $PS1 ] # no prompt? then # non-interactive ... else # interactive ... fi Alternatively, the script can test for the presence of option i in the $- flag. case $- in *i*) # interactive shell ;; *) # non-interactive shell ;; # (Thanks to "UNIX F.A.Q.", 1993) Scripts may be forced to run in interactive mode with the -i option or with a #!/bin/bash -i header. Be aware that this can cause erratic script behavior or show error messages even when no error is present. A wrapper is a shell script that embeds a system command or utility, that saves a set of parameters passed to to that command. Wrapping a script around a complex command line simplifies invoking it. This is expecially useful with sed and awk. A sed script sed sed or awk script awk awk script would normally be invoked from the command line by a sed -e 'commands' or awk 'commands'. Embedding such a script in a Bash script permits calling it more simply, and makes it reusable. This also enables combining the functionality of sed and awk, for example piping the output of a set of sed commands to awk. As a saved executable file, you can then repeatedly invoke it in its original form or modified, without the inconvenience of retyping it on the command line. &ex3; &ex4; &coltotaler; For those scripts needing a single do-it-all tool, a Swiss army knife, there is Perl. Perl combines the capabilities of sed and awk, and throws in a large subset of C, to boot. It is modular and contains support for everything ranging from object-oriented programming up to and including the kitchen sink. Short Perl scripts lend themselves to embedding in shell scripts, and there may even be some substance to the claim that Perl can totally replace shell scripting (though the author of this document remains skeptical). &ex56; It is even possible to combine a Bash script and Perl script within the same file. Depending on how the script is invoked, either the Bash part or the Perl part will execute. &bashandperl; bash$ bash bashandperl.sh Greetings from the Bash part of the script. bash$ perl -x bashandperl.sh Greetings from the Perl part of the script. For tests, the [[ ]] construct may be more appropriate than [ ]. Likewise, arithmetic comparisons might benefit from the (( )) construct. a=8 # All of the comparisons below are equivalent. test "$a" -lt 16 && echo "yes, $a < 16" # "and list" /bin/test "$a" -lt 16 && echo "yes, $a < 16" [ "$a" -lt 16 ] && echo "yes, $a < 16" [[ $a -lt 16 ]] && echo "yes, $a < 16" # Quoting variables within (( a < 16 )) && echo "yes, $a < 16" # [[ ]] and (( )) not necessary. city="New York" # Again, all of the comparisons below are equivalent. test "$city" \< Paris && echo "Yes, Paris is greater than $city" # Greater ASCII order. /bin/test "$city" \< Paris && echo "Yes, Paris is greater than $city" [ "$city" \< Paris ] && echo "Yes, Paris is greater than $city" [[ $city < Paris ]] && echo "Yes, Paris is greater than $city" # Need not quote $city. # Thank you, S.C. Most shell scripts are quick 'n dirty solutions to non-complex problems. As such, optimizing them for speed is not much of an issue. Consider the case, though, where a script carries out an important task, does it well, but runs too slowly. Rewriting it in a compiled language may not be a palatable option. The simplest fix would be to rewrite the parts of the script that slow it down. Is it possible to apply principles of code optimization even to a lowly shell script? Check the loops in the script. Time consumed by repetitive operations adds up quickly. Use the time and times tools to profile computation-intensive commands. Consider rewriting time-critical code sections in C, or even in assembler. Try to minimize file i/o. Bash is not particularly efficient at handling files, so consider using more appropriate tools for this within the script, such as awk or Perl. Try to write your scripts in a structured, coherent form, so they can be reorganized and tightened up as necessary. Some of the optimization techniques applicable to high-level languages may work for scripts, but others, such as loop unrolling, are mostly irrelevant. Above all, use common sense. To keep a record of which user scripts have run during a particular sesssion or over a number of sessions, add the following lines to each script you want to keep track of. This will keep a continuing file record of the script names and invocation times. # Append (>>) following to end of each script tracked. date>> $SAVE_FILE #Date and time. echo $0>> $SAVE_FILE #Script name. echo>> $SAVE_FILE #Blank line as separator. # Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc # (something like ~/.scripts-run) The >> operator appends lines to a file. What if you wish to prepend a line to an existing file, that is, to paste it in at the beginning? file=data.txt title="***This is the title line of data text file***" echo $title | cat - $file >$file.new # "cat -" concatenates stdout to $file. # End result is #+ to write a new file with $title appended at *beginning*. Of course, sed can also do this. A shell script may act as an embedded command inside another shell script, a Tcl or wish script, or even a Makefile. It can be invoked as as an external shell command in a C program using the system() call, i.e., system("script_name");. Put together files containing your favorite and most useful definitions and functions. As necessary, include one or more of these library files in scripts with either the dot (.) or source command. # SCRIPT LIBRARY # ------ ------- # Note: # No "#!" here. # No "live code" either. # Useful variable definitions ROOT_UID=0 # Root has $UID 0. E_NOTROOT=101 # Not root user error. MAXRETVAL=256 # Maximum (positive) return value of a function. SUCCESS=0 FAILURE=-1 # Functions Usage () # "Usage:" message. { if [ -z "$1" ] # No arg passed. then msg=filename else msg=$@ fi echo "Usage: `basename $0` "$msg"" } Check_if_root () # Check if root running script. { # From "ex39.sh" example. if [ "$UID" -ne "$ROOT_UID" ] then echo "Must be root to run this script." exit $E_NOTROOT fi } CreateTempfileName () # Creates a "unique" temp filename. { # From "ex51.sh" example. prefix=temp suffix=`eval date +%s` Tempfilename=$prefix.$suffix } isalpha2 () # Tests whether *entire string* is alphabetic. { # From "isalpha.sh" example. [ $# -eq 1 ] || return $FAILURE case $1 in *[!a-zA-Z]*|"") return $FAILURE;; *) return $SUCCESS;; esac # Thanks, S.C. } abs () # Absolute value. { # Caution: Max return value = 256. E_ARGERR=-999999 if [ -z "$1" ] # Need arg passed. then return $E_ARGERR # Obvious error value returned. fi if [ "$1" -ge 0 ] # If non-negative, then # absval=$1 # stays as-is. else # Otherwise, let "absval = (( 0 - $1 ))" # change sign. fi return $absval } tolower () # Converts string(s) passed as argument(s) { #+ to lowercase. if [ -z "$1" ] # If no argument(s) passed, then #+ send error message echo "(null)" #+ (C-style void-pointer error message) return #+ and return from function. fi echo "$@" | tr A-Z a-z # Translate all passed arguments ($@). return # Use command substitution to set a variable to function output. # For example: # oldvar="A seT of miXed-caSe LEtTerS" # newvar=`tolower "$oldvar"` # echo "$newvar" # a set of mixed-case letters # # Exercise: Rewrite this function to change lowercase passed argument(s) # to uppercase ... toupper() [easy]. } Use special-purpose comment headers to increase clarity and legibility in scripts. ## Caution. rm -rf *.zzy ## The "-rf" options to "rm" are very dangerous, ##+ especially with wildcards. #+ Line continuation. # This is line 1 #+ of a multi-line comment, #+ and this is the final line. #* Note. #o List item. #> Another point of view. while [ "$var1" != "end" ] #> while test "$var1" != "end" Using the $? exit status variable, a script may test if a parameter contains only digits, so it can be treated as an integer. #!/bin/bash SUCCESS=0 E_BADINPUT=65 test "$1" -ne 0 -o "$1" -eq 0 2>/dev/null # An integer is either equal to 0 or not equal to 0. # 2>/dev/null suppresses error message. if [ $? -ne "$SUCCESS" ] then echo "Usage: `basename $0` integer-input" exit $E_BADINPUT fi let "sum = $1 + 25" # Would give error if $1 not integer. echo "Sum = $sum" # Any variable, not just a command line parameter, can be tested this way. exit 0 The 0 - 255 range for function return values is a severe limitation. Global variables and other workarounds are often problematic. An alternative method for a function to communicate a value back to the main body of the script is to have the function write to stdout the return value, and assign this to a variable. &multiplication; The same technique also works for alphanumeric strings. This means that a function can return a non-numeric value. capitalize_ichar () # Capitalizes initial character { #+ of argument string(s) passed. string0="$@" # Accepts multiple arguments. firstchar=${string0:0:1} # First character. string1=${string0:1} # Rest of string(s). FirstChar=`echo "$firstchar" | tr a-z A-Z` # Capitalize first character. echo "$FirstChar$string1" # Output to stdout. } newstring=`capitalize_ichar "each sentence should start with a capital letter."` echo "$newstring" # Each sentence should start with a capital letter. It is even possible for a function to return multiple values with this method. &sumproduct; Next in our bag of trick are techniques for passing an array to a function, then returning an array back to the main body of the script. Passing an array involves loading the space-separated elements of the array into a variable with command substitution. Getting an array back as the return value from a function uses the previously mentioned strategem of echoing the array in the function, then invoking command substitution and the ( ... ) operator to assign it to an array. &arrfunc; For a more elaborate example of passing arrays to functions, see . Using the double parentheses construct, it is possible to use C-like syntax for setting and incrementing variables and in for and while loops. See and . A useful scripting technique is to repeatedly feed the output of a filter (by piping) back to the same filter, but with a different set of arguments and/or options. Especially suitable for this is tr. # From "wstrings.sh" example. wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \ tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '` The run-parts command is handy for running a set of command scripts in sequence, particularly in combination with cron or at. It would be nice to be able to invoke X-Windows widgets from a shell script. There happen to exist several packages that purport to do so, namely Xscript, Xmenu, and widtools. The first two of these no longer seem to be maintained. Fortunately, it is still possible to obtain widtools here. The widtools (widget tools) package requires the XForms library to be installed. Additionally, the Makefile needs some judicious editing before the package will build on a typical Linux system. Finally, three of the six widgets offered do not work (and, in fact, segfault). For more effective scripting with widgets, try Tk or wish (Tcl derivatives), PerlTk (Perl with Tk extensions), tksh (ksh with Tk extensions), XForms4Perl (Perl with XForms extensions), Gtk-Perl (Perl with Gtk extensions), or PyQt (Python with Qt extensions). Can a script recursively call itself? Indeed. &recurse; Too many levels of recursion can exhaust the script's stack space, causing a segfault. This book deals specifically with Bash scripting on a GNU/Linux system. All the same, users of sh and ksh will find much of value here. As it happens, many of the various shells and scripting languages seem to be converging toward the POSIX 1003.2 standard. Invoking Bash with the --posix option or inserting a set -o posix at the head of a script causes Bash to conform very closely to this standard. Even lacking this measure, most Bash scripts will run as-is under ksh, and vice-versa, since Chet Ramey has been busily porting ksh features to the latest versions of Bash. On a commercial UNIX machine, scripts using GNU-specific features of standard commands may not work. This has become less of a problem in the last few years, as the GNU utilities have pretty much displaced their proprietary counterparts even on big-iron UNIX. Caldera's recent release of the source to many of the original UNIX utilities will only accelerate the trend. Even users running that other OS can run UNIX-like shell scripts, and therefore benefit from many of the lessons of this book. The Cygwin package from Cygnus and the MKS utilities from Mortice Kern Associates add shell scripting capabilities to Windows. The current version of Bash, the one you have running on your machine, is actually version 2.XX. bash$ echo $BASH_VERSION 2.04.21(1)-release This update of the classic Bash scripting language added array variables, Chet Ramey promises associative arrays (a Perl feature) in a future Bash release. string and parameter expansion, and a better method of indirect variable references, among other features. &ex77; &ex78; &resistor; &ex79; How did I come to write a Bash scripting book? It's a strange tale. It seems that a couple of years back, I needed to learn shell scripting, and what better way to do that than to read a good book on the subject. I was looking to buy a tutorial and reference covering all aspects of scripting. In fact, I was looking for this very book, or something much like it. Unfortunately, it didn't exist, so if I wanted it, I had to write it. And so, here we are, folks. This reminds me of the apocryphal story about the mad professor. Crazy as a loon, the fellow was. At the sight of a book, any book, at the library, at a bookstore, anywhere, he would become totally obsessed with the idea that he could have written it, should have written it, and done a better job of it to boot. He would thereupon rush home and proceed to do just that, write a book with the same title. When he died some years later, he allegedly had several thousand books to his credit, probably putting even Asimov to shame. The books might not have been any good, who knows, but does that really matter? Here's a fellow who lived his dream, even if he was driven by it, and I can't help admiring the old coot... Who is this guy anyhow? The author claims no credentials or special qualifications, other than a compulsion to write. Those who can, do. Those who can't... get an MCSE. This book is somewhat of a departure from his other major work, HOW-2 Meet Women: The Shy Man's Guide to Relationships. He has also written the Software-Building HOWTO. A Linux user since 1995 (Slackware 2.2, kernel 1.2.1), the author has emitted a few software truffles, including the cruft one-time pad encryption utility, the mcalc mortgage calculator, the judge Scrabble® adjudicator, and the yawl word gaming list package. He got his start in programming using FORTRAN IV on a CDC 3800, but is not the least bit nostalgic for those days. Living in a secluded desert community with wife and dog, he cherishes human frailty. A used IBM Thinkpad, model 760XL laptop (P166, 80 meg RAM) running Red Hat 7.1/7.2. Sure, it's slow and has a funky keyboard, but it beats the heck out of a No. 2 pencil and a Big Chief tablet. Bram Moolenaar's powerful SGML-aware vim text editor. OpenJade, a DSSSL rendering engine for converting SGML documents into other formats. Norman Walsh's DSSSL stylesheets. DocBook, The Definitive Guide, by Norman Walsh and Leonard Muellner (O'Reilly, ISBN 1-56592-580-7). This is the standard reference for anyone attempting to write a document in Docbook SGML format. Community participation made this project possible. The author gratefully acknowledges that writing this book would have been an impossible task without help and feedback from all you people out there. Philippe Martin translated this document into DocBook/SGML. While not on the job at a small French company as a software developer, he enjoys working on GNU/Linux documentation and software, reading literature, playing music, and for his peace of mind making merry with friends. You may run across him somewhere in France or in the Basque Country, or email him at feloy@free.fr. Philippe Martin also pointed out that positional parameters past $9 are possible using {bracket} notation, see . Stephane Chazelas sent a long list of corrections, additions, and example scripts. More than a contributor, he has, in effect, taken on the role of editor for this document. Merci beaucoup ! I would like to especially thank Patrick Callahan, Mike Novak, and Pal Domokos for catching bugs, pointing out ambiguities, and for suggesting clarifications and changes. Their lively discussion of shell scripting and general documentation issues inspired me to try to make this document more readable. I'm grateful to Jim Van Zandt for pointing out errors and omissions in version 0.2 of this document. He also contributed an instructive example script. Many thanks to Jordi Sanfeliu for giving permission to use his fine tree script (). Kudos to Noah Friedman for permission to use his string function script (). Emmanuel Rouat suggested corrections and additions on command substitution and aliases. He also contributed a very nice sample .bashrc file (). Heiner Steven kindly gave permission to use his base conversion script, . He also made a number of corrections and many helpful suggestions. Special thanks. Florian Wisser enlightened me on some of the fine points of testing strings (see ), and on other matters. Oleg Philon sent suggestions concerning cut and pidof. Marc-Jano Knopp sent corrections on DOS batch files. Hyun Jin Cha found several typos in the document in the process of doing a Korean translation. Thanks for pointing these out. Others making helpful suggestions and pointing out errors were Gabor Kiss, Leopold Toetsch, Peter Tillier, Marcus Berglof, Tony Richardson, Nick Drage (script ideas!), Rich Bartell, Jess Thrysoee, Bram Moolenaar, and David Lawyer (himself an author of 4 HOWTOs). My gratitude to Chet Ramey and Brian Fox for writing Bash, an elegant and powerful scripting tool. Thanks most of all to my wife, Anita, for her encouragement and emotional support. DaleDougherty ArnoldRobbins 2nd edition O'Reilly and Associates 1997 1-156592-225-5 To unfold the full power of shell scripting, you need at least a passing familiarity with sed and awk. This is the standard tutorial. It includes an excellent introduction to regular expressions. Read this book. * AeleenFrisch 2nd edition O'Reilly and Associates 1995 1-56592-127-5 This excellent sys admin manual has a decent introduction to shell scripting for sys administrators and does a nice job of explaining the startup and initialization scripts. The book is long overdue for a third edition (are you listening, Tim O'Reilly?). * StephenKochan PatrickWoods Hayden 1990 067248448X The standard reference, though a bit dated by now. * NeilMatthew RichardStones Wrox Press 1996 1874416680 Good in-depth coverage of various programming languages available for Linux, including a fairly strong chapter on shell scripting. * HerbertMayer Windcrest Books 1989 0830693637 Excellent coverage of algorithms and general programming practices. * DavidMedinets McGraw-Hill 1999 0070397333 Good info on shell scripting, with examples, and a short intro to Tcl and Perl. * CameronNewham BillRosenblatt 2nd edition O'Reilly and Associates 1998 1-56592-347-2 This is a valiant effort at a decent shell primer, but somewhat deficient in coverage on programming topics and lacking sufficient examples. * AnatoleOlczak ASP, Inc. 1991 093573922X A very handy pocket reference, despite lacking coverage of Bash-specific features. * JerryPeek TimO'Reilly MikeLoukides 2nd edition O'Reilly and Associates Random House 1997 1-56592-260-3 Contains a couple of sections of very informative in-depth articles on shell programming, but falls short of being a tutorial. It reproduces much of the regular expressions tutorial from the Dougherty and Robbins book, above. * CliffordPickover St. Martin's Press 1990 0-312-04123-3 A treasure trove of ideas and recipes for computer-based exploration of mathematical oddities. * ArnoldRobbins SSC 1998 1-58731-010-5 Excellent Bash pocket reference (don't leave home without it). A bargain at $4.95, but also available for free download on-line in pdf format. * ArnoldRobbins Free Software Foundation / O'Reilly and Associates 2000 1-882114-26-4 The absolute best awk tutorial and reference. The free electronic version of this book is part of the awk documentation, and printed copies of the latest version are available from O'Reilly and Associates. This book has served as an inspiration for the author of this document. * BillRosenblatt O'Reilly and Associates 1993 1-56592-054-6 This well-written book contains some excellent pointers on shell scripting. * PaulSheer 1st edition 2002 0-13-033351-4 Very detailed and readable introduction to Linux system administration. The book is available in print, or on-line. * EllenSiever and the Staff of O'Reilly and Associates 2nd edition O'Reilly and Associates 1999 1-56592-585-8 The all-around best Linux command reference, even has a Bash section. * 2nd edition O'Reilly and Associates 2000 1-56592-815-6 An array of six UNIX books on CD ROM, including UNIX Power Tools, Sed and Awk, and Learning the Korn Shell. A complete set of all the UNIX references and tutorials you would ever need at about $70. Buy this one, even if it means going into debt and not paying the rent. Unfortunately, out of print at present. * The O'Reilly books on Perl. (Actually, any O'Reilly books.) --- Ben Okopnik's well-written introductory Bash scripting articles in issues 53, 54, 55, 57, and 59 of the Linux Gazette , and his explanation of The Deep, Dark Secrets of Bash in issue 56. Chet Ramey's bash - The GNU Shell, a two-part series published in issues 3 and 4 of the Linux Journal, July-August 1994. Mike G's Bash-Programming-Intro HOWTO. Richard's UNIX Scripting Universe. Chet Ramey's Bash F.A.Q. Example shell scripts at Lucc's Shell Scripts . Example shell scripts at SHELLdorado . Example shell scripts at Noah Friedman's script site. Example shell scripts at SourceForge Snippet Library - shell scrips. Giles Orr's Bash-Prompt HOWTO. The sed F.A.Q. Carlos Duarte's instructive Do It With Sed tutorial. The GNU gawk reference manual (gawk is the extended GNU version of awk available on Linux and BSD systems). Trent Fisher's groff tutorial. Mark Komarinski's Printing-Usage HOWTO. There is some nice material on I/O redirection in chapter 10 of the textutils documentation at the University of Alberta site. Rick Hohensee has written the osimpa i386 assembler entirely as Bash scripts. --- The excellent "Bash Reference Manual", by Chet Ramey and Brian Fox, distributed as part of the "bash-2-doc" package (available as an rpm). See especially the instructive example scripts in this package. The comp.os.unix.shell newsgroup. The manpages for bash and bash2, date, expect, expr, find, grep, gzip, ln, patch, tar, tr, bc, xargs. The texinfo documentation on bash, dd, m4, gawk, and sed. These scripts, while not fitting into the text of this document, do illustrate some interesting shell programming techniques. They are useful, too. Have fun analyzing and running them. &manview; &mailformat; This script is a modification of . &rn; &encryptedpw; ©cd; &collatz; &daysbetween; &lifeslow; &gen0data; +++ The following two scripts are by Mark Moraes of the University of Toronto. See the enclosed file Moraes-COPYRIGHT for permissions and restrictions. &behead; &ftpget; + Antek Sawicki contributed the following script, which makes very clever use of the parameter substitution operators discussed in . &pw; + James R. Van Zandt contributed this script, which uses named pipes and, in his words, really exercises quoting and escaping. &fifo; + Stephane Chazelas contributed the following script to demonstrate that generating prime numbers does not require arrays. ℙ + Jordi Sanfeliu gave permission to use his elegant tree script. &tree; Noah Friedman gave permission to use his string function script, which essentially reproduces some of the C-library string manipulation functions. &string; Stephane Chazelas demonstrates object-oriented programming a Bash script. &objoriented; This is a very brief introduction to the sed and awk text processing utilities. We will deal with only a few basic commands here, but that will suffice for understanding simple sed and awk constructs within shell scripts. sed: a non-interactive text file editor awk: a field-oriented pattern processing language with a C-like syntax For all their differences, the two utilities share a similar invocation syntax, both use regular expressions , both read input by default from stdin, and both output to stdout. These are well-behaved UNIX tools, and they work together well. The output from one can be piped into the other, and their combined capabilities give shell scripts some of the power of Perl. One important difference between the utilities is that while shell scripts can easily pass arguments to sed, it is more complicated for awk (see and ). Sed is a non-interactive line editor. It receives text input, whether from stdin or from a file, performs certain operations on specified lines of the input, one line at a time, then outputs the result to stdout or to a file. Within a shell script, sed is usually one of several tool components in a pipe. Sed determines which lines of its input that it will operate on from the address range passed to it. If no address range is specified, the default is all lines. Specify this address range either by line number or by a pattern to match. For example, 3d signals sed to delete line 3 of the input, and /windows/d tells sed that you want every line of the input containing a match to windows deleted. Of all the operations in the sed toolkit, we will focus primarily on the three most commonly used ones. These are printing (to stdout), deletion, and substitution. Operator Name Effect [address-range]/p print Print [specified address range] [address-range]/d delete Delete [specified address range] s/pattern1/pattern2/ substitute Substitute pattern2 for first instance of pattern1 in a line [address-range]/s/pattern1/pattern2/ substitute Substitute pattern2 for first instance of pattern1 in a line, over address-range [address-range]/y/pattern1/pattern2/ transform replace any character in pattern1 with the corresponding character in pattern2, over address-range (equivalent of tr) g global Operate on every pattern match within each matched line of input Unless the g (global) operator is appended to a substitute command, the substitution operates only on the first instance of a pattern match within each line. From the command line and in a shell script, a sed operation may require quoting and certain options. sed -e '/^$/d' $filename # The -e option causes the next string to be interpreted as an editing instruction. # (If passing only a single instruction to "sed", the "-e" is optional.) # The "strong" quotes ('') protect the RE characters in the instruction #+ from reinterpretation as special characters by the body of the script. # (This reserves RE expansion of the instruction for sed.) # # Operates on the text contained in file $filename. Sed uses the -e option to specify that the following string is an instruction or set of instructions. If there is only a single instruction contained in the string, then this option may be omitted. sed -n '/xzy/p' $filename # The -n option tells sed to print only those lines matching the pattern. # Otherwise all input lines would print. # The -e option not necessary here since there is only a single editing instruction. Notation Effect 8d Delete 8th line of input. /^$/d Delete all blank lines. 1,/^$/d Delete from beginning of input up to, and including first blank line. /Jones/p Print only lines containing Jones (with -n option). s/Windows/Linux/ Substitute Linux for first instance ofWindows found in each input line. s/BSOD/stability/g Substitute stability for every instance ofBSOD found in each input line. s/ *$// Delete all spaces at the end of every line. s/00*/0/g Compress all consecutive sequences of zeroes into a single zero. /GUI/d Delete all lines containing GUI. s/GUI//g Delete all instances of GUI, leaving the remainder of each line intact. Substituting a zero-length string for another is equivalent to deleting that string within a line of input. This leaves the remainder of the line intact. Applying s/GUI// to the line The most important parts of any application are its GUI and sound effects results in The most important parts of any application are its and sound effects The backslash represents a newline as a substitution character. In this special case, the replacement expression continues on the next line. s/^ */\ /g This substitution replaces line-beginning spaces with a newline. The net result is to replace paragraph indents with a blank line between paragraphs. An address range followed by one or more operations may require open and closed curly brackets, with appropriate newlines. /[0-9A-Za-z]/,/^$/{ /^$/d } This deletes only the first of each set of consecutive blank lines. That might be useful for single-spacing a text file, but retaining the blank line(s) between paragraphs. A quick way to double-space a text file is sed G filename. For illustrative examples of sed within shell scripts, see: For a more extensive treatment of sed, check the appropriate references in the . Awk is a full-featured text processing language with a syntax reminiscent of C. While it possesses an extensive set of operators and capabilities, we will cover only a couple of these here - the ones most useful for shell scripting. Awk breaks each line of input passed to it into fields. By default, a field is a string of consecutive characters separated by whitespace, though there are options for changing the delimiter. Awk parses and operates on each separate field. This makes awk ideal for handling structured text files, especially tables, data organized into consistent chunks, such as rows and columns. Strong quoting (single quotes) and curly brackets enclose segments of awk code within a shell script. awk '{print $3}' $filename # Prints field #3 of file $filename to stdout. awk '{print $1 $5 $6}' $filename # Prints fields #1, #5, and #6 of file $filename. We have just seen the awk print command in action. The only other feature of awk we need to deal with here is variables. Awk handles variables similarly to shell scripts, though a bit more flexibly. { total += ${column_number} } This adds the value of column_number to the running total of total. Finally, to print total, there is an END command block, executed after the script has processed all its input. END { print total } Corresponding to the END, there is a BEGIN, for a code block to be performed before awk starts processing its input. For examples of awk within shell scripts, see: That's all the awk we'll cover here, folks, but there's lots more to learn. See the appropriate references in the . Exit Code Number Meaning Example Comments 1 catchall for general errors let "var1 = 1/0" miscellaneous errors, such as divide by zero 2 misuse of shell builtins, according to Bash documentation Seldom seen, usually defaults to exit code 1 126 command invoked cannot execute permission problem or command is not an executable 127 command not found possible problem with $PATH or a typo 128 invalid argument to exit exit 3.14159 exit takes only integer args in the range 0 - 255 128+n fatal error signal n kill -9 $PPIDof script $? returns 137 (128 + 9) 130 script terminated by Control-C Control-C is fatal error signal 2, (130 = 128 + 2, see above) 255* exit status out of range exit -1 exit takes only integer args in the range 0 - 255 According to the table, exit codes 1 - 2, 126 - 165, and 255 Out of range exit values can result in unpredictable exit codes. For example, exit 3809 gives an exit code of 225. have special meanings, and should therefore be avoided as user-specified exit parameters. Ending a script with exit 127 would certainly cause confusion when troubleshooting (is the error a command not found or a user-defined one?). However, many scripts use an exit 1 as a general bailout upon error. Since exit code 1 signifies so many possible errors, this might not add any additional ambiguity, but, on the other hand, it probably would not be very informative either. There has been an attempt to systematize exit status numbers (see /usr/include/sysexits.h), but this is intended mostly for C and C++ programmers. It would be well to support a similar standard for scripts. The author of this document proposes restricting user-defined exit codes to the range 64 - 113 (in addition to 0, for success), to conform with the C/C++ standard. This would still leave 50 valid codes, and make troubleshooting scripts more straightforward. All user-defined exit codes in the accompanying examples to this document now conform to this standard, except where overriding circumstances exist, as in . Issuing a $? from the command line after a shell script exits gives results consistent with the table above only from the Bash or sh prompt. Running the C-shell or tcsh may give different values in some cases. written by Stephane Chazelas, and revised by the document author A command expects the first three file descriptors to be available. The first, fd 0 (standard input, stdin), is for reading. The other two (fd 1, stdout and fd 2, stderr) are for writing. There is a stdin, stdout, and a stderr associated with each command. ls 2>&1 means temporarily connecting the stderr of the ls command to the same resource as the shell's stdout. By convention, a command reads its input from fd 0 (stdin), prints normal output to fd 1 (stdout), and error ouput to fd 2 (stderr). If one of those three fd's is not open, you may encounter problems: bash$ cat /etc/passwd >&- cat: standard output: Bad file descriptor For example, when xterm runs, it first initializes itself. Before running the user's shell, xterm opens the terminal device (/dev/pts/<n> or something similar) three times. At this point, Bash inherits these three file descriptors, and each command (child process) run by Bash inherits them in turn, except when you redirect the command. Redirection means reassigning one of the file descriptors to another file (or a pipe, or anything permissable). File descriptors may be reassigned locally (for a command, a command group, a subshell, a while or if or case or for loop...), or globally, for the remainder of the shell (using exec). ls > /dev/null means running ls with its fd 1 connected to /dev/null. bash$ lsof -a -p $$ -d0,1,2 COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME bash 363 bozo 0u CHR 136,1 3 /dev/pts/1 bash 363 bozo 1u CHR 136,1 3 /dev/pts/1 bash 363 bozo 2u CHR 136,1 3 /dev/pts/1 bash$ exec 2> /dev/null bash$ lsof -a -p $$ -d0,1,2 COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME bash 371 bozo 0u CHR 136,1 3 /dev/pts/1 bash 371 bozo 1u CHR 136,1 3 /dev/pts/1 bash 371 bozo 2w CHR 1,3 120 /dev/null bash$ bash -c 'lsof -a -p $$ -d0,1,2' | cat COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME lsof 379 root 0u CHR 136,1 3 /dev/pts/1 lsof 379 root 1w FIFO 0,0 7118 pipe lsof 379 root 2u CHR 136,1 3 /dev/pts/1 bash$ echo "$(bash -c 'lsof -a -p $$ -d0,1,2' 2>&1)" COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME lsof 426 root 0u CHR 136,1 3 /dev/pts/1 lsof 426 root 1w FIFO 0,0 7520 pipe lsof 426 root 2w FIFO 0,0 7520 pipe This works for different types of redirection. Exercise: Analyze the following script. #! /usr/bin/env bash mkfifo /tmp/fifo1 /tmp/fifo2 while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1 exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7) exec 3>&1 ( ( ( while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr | tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 >&7 & exec 3> /tmp/fifo2 echo 1st, to stdout sleep 1 echo 2nd, to stderr >&2 sleep 1 echo 3rd, to fd 3 >&3 sleep 1 echo 4th, to fd 4 >&4 sleep 1 echo 5th, to fd 5 >&5 sleep 1 echo 6th, through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5 sleep 1 echo 7th, to fd 6 >&6 sleep 1 echo 8th, to fd 7 >&7 sleep 1 echo 9th, to fd 8 >&8 ) 4>&1 >&3 3>&- | while read a; do echo "FD4: $a"; done 1>&3 5>&- 6>&- ) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&- ) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&- rm -f /tmp/fifo1 /tmp/fifo2 # For each command and subshell, figure out which fd points to what. exit 0 Localization is an undocumented Bash feature. A localized shell script echoes its text output in the language defined as the system's locale. A Linux user in Berlin, Germany, would get script output in German, whereas his cousin in Berlin, Maryland, would get output from the same script in English. To create a localized script, use the following template to write all messages to the user (error messages, prompts, etc.). #!/bin/bash # localized.sh E_CDERROR=65 error() { printf "$@" >&2 exit $E_CDERROR } cd $var || error $"Can't cd to %s." "$var" read -p $"Enter the value: " var # ... bash$ bash -D localized.sh "Can't cd to %s." "Enter the value: " This lists all the localized text. (The -D option lists double-quoted strings prefixed by a $, without executing the script.) bash$ bash --dump-po-strings localized.sh #: a:6 msgid "Can't cd to %s." msgstr "" #: a:7 msgid "Enter the value: " msgstr "" The --dump-po-strings option to Bash resembles the -D option, but uses gettext po format. Now, build a language.po file for each language that the script will be translated into, specifying the msgstr. As an example: fr.po: #: a:6 msgid "Can't cd to %s." msgstr "Impossible de se positionner dans le répertoire %s." #: a:7 msgid "Enter the value: " msgstr "Entrez la valeur : " Then, run msgfmt. msgfmt -o localized.sh.mo fr.po Place the resulting localized.sh.mo file in the /usr/local/share/locale/fr/LC_MESSAGES directory, and at the beginning of the script, insert the lines: TEXTDOMAINDIR=/usr/local/share/locale TEXTDOMAIN=localized.sh If a user on a French system runs the script, she will get French messages. With older versions of Bash or other shells, localization requires gettext, using the -s option. In this case, the script becomes: #!/bin/bash # localized.sh E_CDERROR=65 error() { local format=$1 shift printf "$(gettext -s "$format")" "$@" >&2 exit $E_CDERROR } cd $var || error "Can't cd to %s." "$var" read -p "$(gettext -s "Enter the value: ")" var # ... The TEXTDOMAIN and TEXTDOMAINDIR variables need to be exported to the environment. --- This appendix written by Stephane Chazelas. The Bash shell provides command-line tools for editing and manipulating a user's command history. This is primarily a convenience, a means of saving keystrokes. Bash history commands: history fc bash$ history 1 mount /mnt/cdrom 2 cd /mnt/cdrom 3 ls ... Internal variables associated with Bash history commands: $HISTCMD $HISTCONTROL $HISTIGNORE $HISTFILE $HISTFILESIZE $HISTSIZE !! !$ !# !N !-N !STRING !?STRING? ^STRING^string^ Unfortunately, the Bash history tools find no use in scripting. #!/bin/bash # history.sh # Attempt to use 'history' command in a script. history # Script produces no output. # History commands do not work within a script. bash$ ./history.sh (no output) The ~/.bashrc file determines the behavior of interactive shells. A good look at this file can lead to a better understanding of Bash. Emmanuel Rouat contributed the following very elaborate .bashrc file, written for a Linux system. He welcomes reader feedback on it. Study the file carefully, and feel free to reuse code snippets and functions from it in your own .bashrc file or even in your scripts. &bashrc; Quite a number of programmers learned scripting on a PC running DOS. Even the crippled DOS batch file language allowed writing some fairly powerful scripts and applications, though they often required extensive kludges and workarounds. Occasionally, the need still arises to convert an old DOS batch file to a UNIX shell script. This is generally not difficult, as DOS batch file operators are only a limited subset of the equivalent shell scripting ones. Batch File Operator Shell Script Equivalent Meaning % $ command-line parameter prefix / - command option flag \ / directory path separator == = (equal-to) string comparison test !==! != (not equal-to) string comparison test | | pipe @ set +v do not echo current command * * filename wild card > > file redirection (overwrite) >> >> file redirection (append) < < redirect stdin %VAR% $VAR environmental variable REM # comment NOT ! negate following test NUL /dev/null black hole for burying command output ECHO echo echo (many more option in Bash) ECHO. echo echo blank line ECHO OFF set +v do not echo command(s) following FOR %%VAR IN (LIST) DO for var in [list]; do for loop :LABEL none (unnecessary) label GOTO none (use a function) jump to another location in the script PAUSE sleep pause or wait an interval CHOICE case or select menu choice IF if if-test IF EXIST FILENAME if [ -e filename ] test if file exists IF !%N==! if [ -z "$N" ] if replaceable parameter N not present CALL source or . (dot operator) include another script COMMAND /C source or . (dot operator) include another script (same as CALL) SET export set an environmental variable SHIFT shift left shift command-line argument list SGN -lt or -gt sign (of integer) ERRORLEVEL $? exit status CON stdin console (stdin) PRN /dev/lp0 (generic) printer device LPT1 /dev/lp0 first printer device COM1 /dev/ttyS0 first serial port Batch files usually contain DOS commands. These must be translated into their UNIX equivalents in order to convert a batch file into a shell script. DOS Command UNIX Equivalent Effect ASSIGN ln link file or directory ATTRIB chmod change file permissions CD cd change directory CHDIR cd change directory CLS clear clear screen COMP diff, comm, cmp file compare COPY cp file copy Ctl-C Ctl-C break (signal) Ctl-Z Ctl-D EOF (end-of-file) DEL rm delete file(s) DELTREE rm -rf delete directory recursively DIR ls -l directory listing ERASE rm delete file(s) EXIT exit exit current process FC comm, cmp file compare FIND grep find strings in files MD mkdir make directory MKDIR mkdir make directory MORE more text file paging filter MOVE mv move PATH $PATH path to executables REN mv rename (move) RENAME mv rename (move) RD rmdir remove directory RMDIR rmdir remove directory SORT sort sort file TIME date display system time TYPE cat output file to stdout XCOPY cp (extended) file copy Virtually all UNIX and shell operators and commands have many more options and enhancements than their DOS and batch file equivalents. Many DOS batch files rely on auxiliary utilities, such as ask.com, a crippled counterpart to read. DOS supports a very limited and incompatible subset of filename wildcard expansion, recognizing only the * and ? characters. Converting a DOS batch file into a shell script is generally straightforward, and the result ofttimes reads better than the original. &VIEWDAT; The script conversion is somewhat of an improvement. &viewdata; Ted Davis' Shell Scripts on the PC site has a set of comprehensive tutorials on the old-fashioned art of batch file programming. Certain of his ingenious techniques could conceivably have relevance for shell scripts. Examine the following script. Run it, then explain what it does. Annotate the script, then rewrite it in a more compact and elegant manner. #!/bin/bash MAX=10000 for((nr=1; nr<$MAX; nr++)) do let "t1 = nr % 5" if [ "$t1" -ne 3 ] then continue fi let "t2 = nr % 7" if [ "$t2" -ne 4 ] then continue fi let "t3 = nr % 9" if [ "$t3" -ne 5 ] then continue fi break # What heppens when you comment out this line? Why? done echo "Number = $nr" exit 0 --- A reader sent in the following code snippet. while read LINE do echo $LINE done < `tail -f /var/log/messages` He wished to write a script tracking changes to the system log file, /var/log/messages. Unfortunately, the above code block hangs and does nothing useful. Why? Fix this so it does work (hint: rather than redirecting the stdin of the loop, try a pipe). --- Analyze , and reorganize it in a simplified and more logical style. See how many of its variables can be eliminated and try to optimize the script to speed up its execution time. Alter the script so that it accepts any ordinary ASCII text file as input for its initial generation. The script will read the first $ROW*$COL characters, and set the occurrences of vowels as living cells. Hint: be sure to translate the spaces in the input file to underscore characters. Write a script to carry out each of the following tasks. Home Directory Listing Perform a recursive directory listing on the user's home directory and save the information to a file. Compress the file, have the script prompt the user to insert a floppy, then press ENTER. Finally, save the file to the floppy. Converting for loops to while and until loops Convert the for loops in to while loops. Hint: store the data in an array and step through the array elements. Having already done the heavy lifting, now convert the loops in the example to until loops. Changing the line spacing of a text file Write a script that reads each line of a target file, then writes the line back to stdout, but with an extra blank line following. This has the effect of double-spacing the file. Include all necessary code to check whether the script gets the necessary command line argument (a filename), and whether the specified file exists. When the script runs correctly, modify it to triple-space the target file. Finally, write a script to remove all blank lines from the target file, single-spacing it. Backwards Listing Write a script that echoes itself to stdout, but backwards. Primes Print (to stdout) all prime numbers between 60000 and 63000. The output should be nicely formatted in columns (hint: use printf). Unique System ID Generate a unique 6-digit hexadecimal identifier for your computer. Do not use the flawed hostid command. Hint: md5sum /etc/passwd, then select the first 6 digits of output. Backup Archive as a tarball (*.tar.gz file) all the files in your home directory tree (/home/your-name) that have been modified in the last 24 hours. Hint: use find. Safe Delete Write, as a script, a safe delete command, srm.sh. Filenames passed as command-line arguments to this script are not deleted, but instead gzipped and moved to a /home/username/trash directory. At invocation, the script checks the trash directory for files older than 48 hours and deletes them. Managing Disk Space List, one at a time, all files larger than 100K in the /home/username directory tree. Give the user the option to delete or compress the file, then proceed to show the next one. Write to a logfile the names of all deleted files and the deletion times. Making Change What is the most efficient way to make change for $1.68, using only coins in common circulations (up to 25c)? It's 6 quarters, 1 dime, a nickel, and three cents. Given any arbitrary command line input in dollars and cents ($*.??), calculate the change, using the minimum number of coins. If your home country is not the United States, you may use your local currency units instead. The script will need to parse the command line input, then change it to multiples of the smallest monetary unit (cents or whatever). Hint: look at . Quadratic Equations Solve a quadratic equation of the form Ax^2 + Bx + C = 0. Have a script take as arguments the coefficients, A, B, and C, and return the solutions to four decimal places. Hint: pipe the coefficients to bc, using the well-known formula, x = ( -B +/- sqrt( B^2 - 4AC ) ) / 2A. Lucky Numbers A "lucky number" is one whose individual digits add up to 7, in successive additions. For example, 62431 is a "lucky number" (6 + 2 + 4 + 3 + 1 = 16, 1 + 6 = 7). Find all the "lucky numbers" between 1000 and 10000. Alphabetizing a String Alphabetize (in ASCII order) an arbitrary string read from the command line. Parsing Parse /etc/passwd, and output its contents in nice, easy-to-read tabular form. Pretty-Printing a Data File Certain database and spreadsheet packages use save-files with comma-separated values (CSVs). Other applications often need to parse these files. Given a data file with comma-separated fields, of the form: Jones,Bill,235 S. Williams St.,Denver,CO,80221,(303) 244-7989 Smith,Tom,404 Polk Ave.,Los Angeles,CA,90003,(213) 879-5612 ... Reformat the data and print it out to stdout in labeled, evenly-spaced columns. Logging File Accesses Log all accesses to the files in /etc during the course of a single day. This information should include the filename, user name, and access time. If any alterations to the files take place, that should be flagged. Write this data as neatly formatted records in a logfile. Strip Comments Strip all comments from a shell script whose name is specified on the command line. Note that the #! line must not be stripped out. HTML Conversion Convert a given text file to HTML. This non-interactive script automatically inserts all appropriate HTML tags into a file specified as an argument. Strip HTML Tags Strip all HTML tags from a specified HTML file, then reformat it into lines between 60 and 75 characters in length. Reset paragraph and block spacing, as appropriate, and convert HTML tables to their approximate text equivalent. XML Conversion Convert an XML file to both HTML and text form. Morse Code Convert a text file to Morse code. Each character of the text file will be represented as a corresponding Morse code group of dots and dashes (underscores), separated by whitespace from the next. For example, script ===> ... _._. ._. .. .__. _. Hex Dump Do a hex(adecimal) dump on a binary file specified as an argument. The output should be in neat tabular fields, with the first field showing the address, each of the next 8 fields a 4-byte hex number, and the final field the ASCII equivalent of the previous 8 fields. Emulating a Shift Register Using as an inspiration, write a script that emulates a 64-bit shift register as an array. Implement functions to load the register, shift left, and shift right. Finally, write a function that interprets the register contents as eight 8-bit ASCII characters. Determinant Solve a 4 x 4 determinant. Hidden Words Write a word-find puzzle generator, a script that hides 10 input words in a 10 x 10 matrix of random letters. The words may be hidden across, down, or diagonally. Anagramming Anagram 4-letter input. For example, the anagrams of word are: do or rod row word. You may use /usr/share/dict/linux.words as the reference list. Playfair Cipher Implement the Playfair (Wheatstone) Cipher in a script. The Playfair Cipher encrypts text by substitution of each 2-letter digram (grouping). Traditionally, one would use a 5 x 5 letter scrambled alphabet code key square for the encryption and decryption. C O D E S A B F G H I K L M N P Q R T U V W X Y Z Each letter of the alphabet appears once, except "I" also represents "J". The arbitrarily chosen key word, "CODES" comes first, then all the rest of the alphabet, skipping letters already used. To encrypt, separate the plaintext message into digrams (2-letter groups). If a group has two identical letters, delete the second, and form a new group. If there is a single letter left over at the end, insert a "null" character, typically an "X". THIS IS A TOP SECRET MESSAGE TH IS IS AT OP SE CR ET ME SA GE For each digram, there are three possibilities. ---------------------------------------------- 1) Both letters will be on the same row of the key square For each letter, substitute the one immediately to the right, in that row. If necessary, wrap around left to the beginning of the row. or 2) Both letters will be in the same column of the key square For each letter, substitute the one immediately below it, in that row. If necessary, wrap around to the top of the column. or 3) Both letters will form the corners of a rectangle within the key square. For each letter, substitute the one on the other corner the rectangle which lies on the same row. The "TH" digram falls under case #3. G H M N T U (Rectangle with "T" and "H" at corners) T --> U H --> G The "SE" digram falls under case #1. C O D E S (Row containing "S" and "E") S --> C (wraps around left to beginning of row) E --> S ========================================================================= To decrypt encrypted text, reverse the above procedure under cases #1 and #2 (move in opposite direction for substitution). Under case #3, just take the remaining two corners of the rectangle. Helen Fouche Gaines' classic work, "Elementary Cryptoanalysis" (1939), gives a fairly detailed rundown on the Playfair Cipher and its solution methods. This script will have three main sections Generating the key square, based on a user-input keyword. Encrypting a plaintext message. Decrypting encrypted text. The script will make extensive use of arrays and functions. -- Please do not send the author your solutions to these exercises. There are better ways to impress him with your cleverness, such as submitting bugfixes and suggestions for improving this book. The Advanced Bash-Scripting Guide is copyright, (c) 2000, by Mendel Cooper. This document may only be distributed subject to the terms and conditions set forth in the LDP License These are very liberal terms, and they should not hinder any legitimate distribution or use of this book. The author especially encourages the use of this book for instructional purposes. Essentially, you may freely distribute this book in unaltered electronic form. You must obtain the author's permission to distribute a modified version or derivative work. The purpose of this restriction is to preserve the artistic integrity of this document and to prevent forking. The commercial print rights to this book are available. Please notify the author if interested. --- Hyun Jin Cha has done a Korean translation of version 1.0.11 of this book. Spanish, Portuguese, French, and Chinese translations are underway. If you wish to translate this document into another language, please feel free to do so, subject to the terms stated above. The author wishes to be notified of such efforts.